Calibre Web Epub Downloading + Calibre Wireless Device Syncing (#219)

## Summary

Adds support for browsing and downloading books from a Calibre-web
server via OPDS.
How it works
1. Configure server URL in Settings → Calibre Web URL (e.g.,
https://myserver.com:port I use Cloudflare tunnel to make my server
accessible anywhere fwiw)
2. "Calibre Library" will now show on the the home screen
3. Browse the catalog - navigate through categories like "By Newest",
"By Author", "By Series", etc.
4. Download books - select a book and press Confirm to download the EPUB
to your device
Navigation
- Up/Down - Move through entries
- Confirm - Open folder or download book
- Back - Go to parent catalog, or exit to home if at root
- Navigation entries show with > prefix, books show title and author
- Button hints update dynamically ("Open" for folders, "Download" for
books)
Technical details
- Fetches OPDS catalog from {server_url}/opds
- Parses both navigation feeds (catalog links) and acquisition feeds
(downloadable books)
- Maintains navigation history stack for back navigation
- Handles absolute paths in OPDS links correctly (e.g.,
/books/opds/navcatalog/...)
- Downloads EPUBs directly to the SD card root
Note
The server URL should be typed to include https:// if the server
requires it - HTTP→HTTPS redirects may cause SSL errors on ESP32.

## Additional Context

* I also changed the home titles to use uppercase for each word and
added a setting to change the size of the side margins

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

lib/OpdsParser/OpdsParser.cpp

@@ -0,0 +1,219 @@
+#include "OpdsParser.h"
+
+#include <HardwareSerial.h>
+
+#include <cstring>
+
+OpdsParser::~OpdsParser() {
+  if (parser) {
+    XML_StopParser(parser, XML_FALSE);
+    XML_SetElementHandler(parser, nullptr, nullptr);
+    XML_SetCharacterDataHandler(parser, nullptr);
+    XML_ParserFree(parser);
+    parser = nullptr;
+  }
+}
+
+bool OpdsParser::parse(const char* xmlData, const size_t length) {
+  clear();
+
+  parser = XML_ParserCreate(nullptr);
+  if (!parser) {
+    Serial.printf("[%lu] [OPDS] Couldn't allocate memory for parser\n", millis());
+    return false;
+  }
+
+  XML_SetUserData(parser, this);
+  XML_SetElementHandler(parser, startElement, endElement);
+  XML_SetCharacterDataHandler(parser, characterData);
+
+  // Parse in chunks to avoid large buffer allocations
+  const char* currentPos = xmlData;
+  size_t remaining = length;
+  constexpr size_t chunkSize = 1024;
+
+  while (remaining > 0) {
+    void* const buf = XML_GetBuffer(parser, chunkSize);
+    if (!buf) {
+      Serial.printf("[%lu] [OPDS] Couldn't allocate memory for buffer\n", millis());
+      XML_ParserFree(parser);
+      parser = nullptr;
+      return false;
+    }
+
+    const size_t toRead = remaining < chunkSize ? remaining : chunkSize;
+    memcpy(buf, currentPos, toRead);
+
+    const bool isFinal = (remaining == toRead);
+    if (XML_ParseBuffer(parser, static_cast<int>(toRead), isFinal) == XML_STATUS_ERROR) {
+      Serial.printf("[%lu] [OPDS] Parse error at line %lu: %s\n", millis(), XML_GetCurrentLineNumber(parser),
+                    XML_ErrorString(XML_GetErrorCode(parser)));
+      XML_ParserFree(parser);
+      parser = nullptr;
+      return false;
+    }
+
+    currentPos += toRead;
+    remaining -= toRead;
+  }
+
+  // Clean up parser
+  XML_ParserFree(parser);
+  parser = nullptr;
+
+  Serial.printf("[%lu] [OPDS] Parsed %zu entries\n", millis(), entries.size());
+  return true;
+}
+
+void OpdsParser::clear() {
+  entries.clear();
+  currentEntry = OpdsEntry{};
+  currentText.clear();
+  inEntry = false;
+  inTitle = false;
+  inAuthor = false;
+  inAuthorName = false;
+  inId = false;
+}
+
+std::vector<OpdsEntry> OpdsParser::getBooks() const {
+  std::vector<OpdsEntry> books;
+  for (const auto& entry : entries) {
+    if (entry.type == OpdsEntryType::BOOK) {
+      books.push_back(entry);
+    }
+  }
+  return books;
+}
+
+const char* OpdsParser::findAttribute(const XML_Char** atts, const char* name) {
+  for (int i = 0; atts[i]; i += 2) {
+    if (strcmp(atts[i], name) == 0) {
+      return atts[i + 1];
+    }
+  }
+  return nullptr;
+}
+
+void XMLCALL OpdsParser::startElement(void* userData, const XML_Char* name, const XML_Char** atts) {
+  auto* self = static_cast<OpdsParser*>(userData);
+
+  // Check for entry element (with or without namespace prefix)
+  if (strcmp(name, "entry") == 0 || strstr(name, ":entry") != nullptr) {
+    self->inEntry = true;
+    self->currentEntry = OpdsEntry{};
+    return;
+  }
+
+  if (!self->inEntry) return;
+
+  // Check for title element
+  if (strcmp(name, "title") == 0 || strstr(name, ":title") != nullptr) {
+    self->inTitle = true;
+    self->currentText.clear();
+    return;
+  }
+
+  // Check for author element
+  if (strcmp(name, "author") == 0 || strstr(name, ":author") != nullptr) {
+    self->inAuthor = true;
+    return;
+  }
+
+  // Check for author name element
+  if (self->inAuthor && (strcmp(name, "name") == 0 || strstr(name, ":name") != nullptr)) {
+    self->inAuthorName = true;
+    self->currentText.clear();
+    return;
+  }
+
+  // Check for id element
+  if (strcmp(name, "id") == 0 || strstr(name, ":id") != nullptr) {
+    self->inId = true;
+    self->currentText.clear();
+    return;
+  }
+
+  // Check for link element
+  if (strcmp(name, "link") == 0 || strstr(name, ":link") != nullptr) {
+    const char* rel = findAttribute(atts, "rel");
+    const char* type = findAttribute(atts, "type");
+    const char* href = findAttribute(atts, "href");
+
+    if (href) {
+      // Check for acquisition link with epub type (this is a downloadable book)
+      if (rel && type && strstr(rel, "opds-spec.org/acquisition") != nullptr &&
+          strcmp(type, "application/epub+zip") == 0) {
+        self->currentEntry.type = OpdsEntryType::BOOK;
+        self->currentEntry.href = href;
+      }
+      // Check for navigation link (subsection or no rel specified with atom+xml type)
+      else if (type && strstr(type, "application/atom+xml") != nullptr) {
+        // Only set navigation link if we don't already have an epub link
+        if (self->currentEntry.type != OpdsEntryType::BOOK) {
+          self->currentEntry.type = OpdsEntryType::NAVIGATION;
+          self->currentEntry.href = href;
+        }
+      }
+    }
+  }
+}
+
+void XMLCALL OpdsParser::endElement(void* userData, const XML_Char* name) {
+  auto* self = static_cast<OpdsParser*>(userData);
+
+  // Check for entry end
+  if (strcmp(name, "entry") == 0 || strstr(name, ":entry") != nullptr) {
+    // Only add entry if it has required fields (title and href)
+    if (!self->currentEntry.title.empty() && !self->currentEntry.href.empty()) {
+      self->entries.push_back(self->currentEntry);
+    }
+    self->inEntry = false;
+    self->currentEntry = OpdsEntry{};
+    return;
+  }
+
+  if (!self->inEntry) return;
+
+  // Check for title end
+  if (strcmp(name, "title") == 0 || strstr(name, ":title") != nullptr) {
+    if (self->inTitle) {
+      self->currentEntry.title = self->currentText;
+    }
+    self->inTitle = false;
+    return;
+  }
+
+  // Check for author end
+  if (strcmp(name, "author") == 0 || strstr(name, ":author") != nullptr) {
+    self->inAuthor = false;
+    return;
+  }
+
+  // Check for author name end
+  if (self->inAuthor && (strcmp(name, "name") == 0 || strstr(name, ":name") != nullptr)) {
+    if (self->inAuthorName) {
+      self->currentEntry.author = self->currentText;
+    }
+    self->inAuthorName = false;
+    return;
+  }
+
+  // Check for id end
+  if (strcmp(name, "id") == 0 || strstr(name, ":id") != nullptr) {
+    if (self->inId) {
+      self->currentEntry.id = self->currentText;
+    }
+    self->inId = false;
+    return;
+  }
+}
+
+void XMLCALL OpdsParser::characterData(void* userData, const XML_Char* s, const int len) {
+  auto* self = static_cast<OpdsParser*>(userData);
+
+  // Only accumulate text when in a text element
+  if (self->inTitle || self->inAuthorName || self->inId) {
+    self->currentText.append(s, len);
+  }
+}

lib/OpdsParser/OpdsParser.h

@@ -0,0 +1,99 @@
+#pragma once
+#include <expat.h>
+
+#include <string>
+#include <vector>
+
+/**
+ * Type of OPDS entry.
+ */
+enum class OpdsEntryType {
+  NAVIGATION,  // Link to another catalog
+  BOOK         // Downloadable book
+};
+
+/**
+ * Represents an entry from an OPDS feed (either a navigation link or a book).
+ */
+struct OpdsEntry {
+  OpdsEntryType type = OpdsEntryType::NAVIGATION;
+  std::string title;
+  std::string author;  // Only for books
+  std::string href;    // Navigation URL or epub download URL
+  std::string id;
+};
+
+// Legacy alias for backward compatibility
+using OpdsBook = OpdsEntry;
+
+/**
+ * Parser for OPDS (Open Publication Distribution System) Atom feeds.
+ * Uses the Expat XML parser to parse OPDS catalog entries.
+ *
+ * Usage:
+ *   OpdsParser parser;
+ *   if (parser.parse(xmlData, xmlLength)) {
+ *     for (const auto& entry : parser.getEntries()) {
+ *       if (entry.type == OpdsEntryType::BOOK) {
+ *         // Downloadable book
+ *       } else {
+ *         // Navigation link to another catalog
+ *       }
+ *     }
+ *   }
+ */
+class OpdsParser {
+ public:
+  OpdsParser() = default;
+  ~OpdsParser();
+
+  // Disable copy
+  OpdsParser(const OpdsParser&) = delete;
+  OpdsParser& operator=(const OpdsParser&) = delete;
+
+  /**
+   * Parse an OPDS XML feed.
+   * @param xmlData Pointer to the XML data
+   * @param length Length of the XML data
+   * @return true if parsing succeeded, false on error
+   */
+  bool parse(const char* xmlData, size_t length);
+
+  /**
+   * Get the parsed entries (both navigation and book entries).
+   * @return Vector of OpdsEntry entries
+   */
+  const std::vector<OpdsEntry>& getEntries() const { return entries; }
+
+  /**
+   * Get only book entries (legacy compatibility).
+   * @return Vector of book entries
+   */
+  std::vector<OpdsEntry> getBooks() const;
+
+  /**
+   * Clear all parsed entries.
+   */
+  void clear();
+
+ private:
+  // Expat callbacks
+  static void XMLCALL startElement(void* userData, const XML_Char* name, const XML_Char** atts);
+  static void XMLCALL endElement(void* userData, const XML_Char* name);
+  static void XMLCALL characterData(void* userData, const XML_Char* s, int len);
+
+  // Helper to find attribute value
+  static const char* findAttribute(const XML_Char** atts, const char* name);
+
+  XML_Parser parser = nullptr;
+  std::vector<OpdsEntry> entries;
+  OpdsEntry currentEntry;
+  std::string currentText;
+
+  // Parser state
+  bool inEntry = false;
+  bool inTitle = false;
+  bool inAuthor = false;
+  bool inAuthorName = false;
+  bool inId = false;
+};

src/CrossPointSettings.cpp

@@ -4,6 +4,8 @@
 #include <SDCardManager.h>
 #include <Serialization.h>
 
+#include <cstring>
+
 #include "fontIds.h"
 
 // Initialize the static instance
@@ -12,7 +14,7 @@ CrossPointSettings CrossPointSettings::instance;
 namespace {
 constexpr uint8_t SETTINGS_FILE_VERSION = 1;
 // Increment this when adding new persisted settings fields
-constexpr uint8_t SETTINGS_COUNT = 15;
+constexpr uint8_t SETTINGS_COUNT = 16;
 constexpr char SETTINGS_FILE[] = "/.crosspoint/settings.bin";
 }  // namespace
 
@@ -40,6 +42,7 @@ bool CrossPointSettings::saveToFile() const {
   serialization::writePod(outputFile, paragraphAlignment);
   serialization::writePod(outputFile, sleepTimeout);
   serialization::writePod(outputFile, refreshFrequency);
+  serialization::writeString(outputFile, std::string(opdsServerUrl));
   serialization::writePod(outputFile, screenMargin);
   serialization::writePod(outputFile, sleepScreenCoverMode);
   outputFile.close();
@@ -94,10 +97,16 @@ bool CrossPointSettings::loadFromFile() {
     if (++settingsRead >= fileSettingsCount) break;
     serialization::readPod(inputFile, refreshFrequency);
     if (++settingsRead >= fileSettingsCount) break;
+    {
+      std::string urlStr;
+      serialization::readString(inputFile, urlStr);
+      strncpy(opdsServerUrl, urlStr.c_str(), sizeof(opdsServerUrl) - 1);
+      opdsServerUrl[sizeof(opdsServerUrl) - 1] = '\0';
+    }
+    if (++settingsRead >= fileSettingsCount) break;
     serialization::readPod(inputFile, screenMargin);
     if (++settingsRead >= fileSettingsCount) break;
     serialization::readPod(inputFile, sleepScreenCoverMode);
-    if (++settingsRead >= fileSettingsCount) break;
   } while (false);
 
   inputFile.close();

src/CrossPointSettings.h

@@ -77,6 +77,8 @@ class CrossPointSettings {
   uint8_t sleepTimeout = SLEEP_10_MIN;
   // E-ink refresh frequency (default 15 pages)
   uint8_t refreshFrequency = REFRESH_15;
+  // OPDS browser settings
+  char opdsServerUrl[128] = "";
 
   // Reader screen margin settings
   uint8_t screenMargin = 5;
@@ -95,7 +97,6 @@ class CrossPointSettings {
   float getReaderLineCompression() const;
   unsigned long getSleepTimeoutMs() const;
   int getRefreshFrequency() const;
-  int getReaderScreenMargin() const;
 };
 
 // Helper macro to access settings

src/ScreenComponents.cpp

@@ -2,6 +2,7 @@
 
 #include <GfxRenderer.h>
 
+#include <cstdint>
 #include <string>
 
 #include "Battery.h"
@@ -39,3 +40,26 @@ void ScreenComponents::drawBattery(const GfxRenderer& renderer, const int left,
 
   renderer.fillRect(x + 2, y + 2, filledWidth, batteryHeight - 4);
 }
+
+void ScreenComponents::drawProgressBar(const GfxRenderer& renderer, const int x, const int y, const int width,
+                                       const int height, const size_t current, const size_t total) {
+  if (total == 0) {
+    return;
+  }
+
+  // Use 64-bit arithmetic to avoid overflow for large files
+  const int percent = static_cast<int>((static_cast<uint64_t>(current) * 100) / total);
+
+  // Draw outline
+  renderer.drawRect(x, y, width, height);
+
+  // Draw filled portion
+  const int fillWidth = (width - 4) * percent / 100;
+  if (fillWidth > 0) {
+    renderer.fillRect(x + 2, y + 2, fillWidth, height - 4);
+  }
+
+  // Draw percentage text centered below bar
+  const std::string percentText = std::to_string(percent) + "%";
+  renderer.drawCenteredText(UI_10_FONT_ID, y + height + 15, percentText.c_str());
+}

src/ScreenComponents.h

@@ -1,8 +1,24 @@
 #pragma once
 
+#include <cstddef>
+#include <cstdint>
+
 class GfxRenderer;
 
 class ScreenComponents {
  public:
   static void drawBattery(const GfxRenderer& renderer, int left, int top);
+
+  /**
+   * Draw a progress bar with percentage text.
+   * @param renderer The graphics renderer
+   * @param x Left position of the bar
+   * @param y Top position of the bar
+   * @param width Width of the bar
+   * @param height Height of the bar
+   * @param current Current progress value
+   * @param total Total value for 100% progress
+   */
+  static void drawProgressBar(const GfxRenderer& renderer, int x, int y, int width, int height, size_t current,
+                              size_t total);
 };

src/activities/browser/OpdsBookBrowserActivity.cpp

@@ -0,0 +1,398 @@
+#include "OpdsBookBrowserActivity.h"
+
+#include <GfxRenderer.h>
+#include <HardwareSerial.h>
+#include <WiFi.h>
+
+#include "CrossPointSettings.h"
+#include "MappedInputManager.h"
+#include "ScreenComponents.h"
+#include "WifiCredentialStore.h"
+#include "fontIds.h"
+#include "network/HttpDownloader.h"
+#include "util/StringUtils.h"
+#include "util/UrlUtils.h"
+
+namespace {
+constexpr int PAGE_ITEMS = 23;
+constexpr int SKIP_PAGE_MS = 700;
+constexpr char OPDS_ROOT_PATH[] = "opds";  // No leading slash - relative to server URL
+}  // namespace
+
+void OpdsBookBrowserActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<OpdsBookBrowserActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void OpdsBookBrowserActivity::onEnter() {
+  Activity::onEnter();
+
+  renderingMutex = xSemaphoreCreateMutex();
+  state = BrowserState::CHECK_WIFI;
+  entries.clear();
+  navigationHistory.clear();
+  currentPath = OPDS_ROOT_PATH;
+  selectorIndex = 0;
+  errorMessage.clear();
+  statusMessage = "Checking WiFi...";
+  updateRequired = true;
+
+  xTaskCreate(&OpdsBookBrowserActivity::taskTrampoline, "OpdsBookBrowserTask",
+              4096,               // Stack size (larger for HTTP operations)
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+
+  // Check WiFi and connect if needed, then fetch feed
+  checkAndConnectWifi();
+}
+
+void OpdsBookBrowserActivity::onExit() {
+  Activity::onExit();
+
+  // Turn off WiFi when exiting
+  WiFi.mode(WIFI_OFF);
+
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+  entries.clear();
+  navigationHistory.clear();
+}
+
+void OpdsBookBrowserActivity::loop() {
+  // Handle error state - Confirm retries, Back goes back or home
+  if (state == BrowserState::ERROR) {
+    if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+      state = BrowserState::LOADING;
+      statusMessage = "Loading...";
+      updateRequired = true;
+      fetchFeed(currentPath);
+    } else if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+      navigateBack();
+    }
+    return;
+  }
+
+  // Handle WiFi check state - only Back works
+  if (state == BrowserState::CHECK_WIFI) {
+    if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+      onGoHome();
+    }
+    return;
+  }
+
+  // Handle loading state - only Back works
+  if (state == BrowserState::LOADING) {
+    if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+      navigateBack();
+    }
+    return;
+  }
+
+  // Handle downloading state - no input allowed
+  if (state == BrowserState::DOWNLOADING) {
+    return;
+  }
+
+  // Handle browsing state
+  if (state == BrowserState::BROWSING) {
+    const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::Up) ||
+                              mappedInput.wasReleased(MappedInputManager::Button::Left);
+    const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::Down) ||
+                              mappedInput.wasReleased(MappedInputManager::Button::Right);
+    const bool skipPage = mappedInput.getHeldTime() > SKIP_PAGE_MS;
+
+    if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+      if (!entries.empty()) {
+        const auto& entry = entries[selectorIndex];
+        if (entry.type == OpdsEntryType::BOOK) {
+          downloadBook(entry);
+        } else {
+          navigateToEntry(entry);
+        }
+      }
+    } else if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+      navigateBack();
+    } else if (prevReleased && !entries.empty()) {
+      if (skipPage) {
+        selectorIndex = ((selectorIndex / PAGE_ITEMS - 1) * PAGE_ITEMS + entries.size()) % entries.size();
+      } else {
+        selectorIndex = (selectorIndex + entries.size() - 1) % entries.size();
+      }
+      updateRequired = true;
+    } else if (nextReleased && !entries.empty()) {
+      if (skipPage) {
+        selectorIndex = ((selectorIndex / PAGE_ITEMS + 1) * PAGE_ITEMS) % entries.size();
+      } else {
+        selectorIndex = (selectorIndex + 1) % entries.size();
+      }
+      updateRequired = true;
+    }
+  }
+}
+
+void OpdsBookBrowserActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void OpdsBookBrowserActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  const auto pageHeight = renderer.getScreenHeight();
+
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, "Calibre Library", true, EpdFontFamily::BOLD);
+
+  if (state == BrowserState::CHECK_WIFI) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, statusMessage.c_str());
+    const auto labels = mappedInput.mapLabels("« Back", "", "", "");
+    renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+    renderer.displayBuffer();
+    return;
+  }
+
+  if (state == BrowserState::LOADING) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, statusMessage.c_str());
+    const auto labels = mappedInput.mapLabels("« Back", "", "", "");
+    renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+    renderer.displayBuffer();
+    return;
+  }
+
+  if (state == BrowserState::ERROR) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, "Error:");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, errorMessage.c_str());
+    const auto labels = mappedInput.mapLabels("« Back", "Retry", "", "");
+    renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+    renderer.displayBuffer();
+    return;
+  }
+
+  if (state == BrowserState::DOWNLOADING) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 40, "Downloading...");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 10, statusMessage.c_str());
+    if (downloadTotal > 0) {
+      const int barWidth = pageWidth - 100;
+      constexpr int barHeight = 20;
+      constexpr int barX = 50;
+      const int barY = pageHeight / 2 + 20;
+      ScreenComponents::drawProgressBar(renderer, barX, barY, barWidth, barHeight, downloadProgress, downloadTotal);
+    }
+    renderer.displayBuffer();
+    return;
+  }
+
+  // Browsing state
+  // Show appropriate button hint based on selected entry type
+  const char* confirmLabel = "Open";
+  if (!entries.empty() && entries[selectorIndex].type == OpdsEntryType::BOOK) {
+    confirmLabel = "Download";
+  }
+  const auto labels = mappedInput.mapLabels("« Back", confirmLabel, "", "");
+  renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  if (entries.empty()) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, "No entries found");
+    renderer.displayBuffer();
+    return;
+  }
+
+  const auto pageStartIndex = selectorIndex / PAGE_ITEMS * PAGE_ITEMS;
+  renderer.fillRect(0, 60 + (selectorIndex % PAGE_ITEMS) * 30 - 2, pageWidth - 1, 30);
+
+  for (size_t i = pageStartIndex; i < entries.size() && i < static_cast<size_t>(pageStartIndex + PAGE_ITEMS); i++) {
+    const auto& entry = entries[i];
+
+    // Format display text with type indicator
+    std::string displayText;
+    if (entry.type == OpdsEntryType::NAVIGATION) {
+      displayText = "> " + entry.title;  // Folder/navigation indicator
+    } else {
+      // Book: "Title - Author" or just "Title"
+      displayText = entry.title;
+      if (!entry.author.empty()) {
+        displayText += " - " + entry.author;
+      }
+    }
+
+    auto item = renderer.truncatedText(UI_10_FONT_ID, displayText.c_str(), renderer.getScreenWidth() - 40);
+    renderer.drawText(UI_10_FONT_ID, 20, 60 + (i % PAGE_ITEMS) * 30, item.c_str(),
+                      i != static_cast<size_t>(selectorIndex));
+  }
+
+  renderer.displayBuffer();
+}
+
+void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
+  const char* serverUrl = SETTINGS.opdsServerUrl;
+  if (strlen(serverUrl) == 0) {
+    state = BrowserState::ERROR;
+    errorMessage = "No server URL configured";
+    updateRequired = true;
+    return;
+  }
+
+  std::string url = UrlUtils::buildUrl(serverUrl, path);
+  Serial.printf("[%lu] [OPDS] Fetching: %s\n", millis(), url.c_str());
+
+  std::string content;
+  if (!HttpDownloader::fetchUrl(url, content)) {
+    state = BrowserState::ERROR;
+    errorMessage = "Failed to fetch feed";
+    updateRequired = true;
+    return;
+  }
+
+  OpdsParser parser;
+  if (!parser.parse(content.c_str(), content.size())) {
+    state = BrowserState::ERROR;
+    errorMessage = "Failed to parse feed";
+    updateRequired = true;
+    return;
+  }
+
+  entries = parser.getEntries();
+  selectorIndex = 0;
+
+  if (entries.empty()) {
+    state = BrowserState::ERROR;
+    errorMessage = "No entries found";
+    updateRequired = true;
+    return;
+  }
+
+  state = BrowserState::BROWSING;
+  updateRequired = true;
+}
+
+void OpdsBookBrowserActivity::navigateToEntry(const OpdsEntry& entry) {
+  // Push current path to history before navigating
+  navigationHistory.push_back(currentPath);
+  currentPath = entry.href;
+
+  state = BrowserState::LOADING;
+  statusMessage = "Loading...";
+  entries.clear();
+  selectorIndex = 0;
+  updateRequired = true;
+
+  fetchFeed(currentPath);
+}
+
+void OpdsBookBrowserActivity::navigateBack() {
+  if (navigationHistory.empty()) {
+    // At root, go home
+    onGoHome();
+  } else {
+    // Go back to previous catalog
+    currentPath = navigationHistory.back();
+    navigationHistory.pop_back();
+
+    state = BrowserState::LOADING;
+    statusMessage = "Loading...";
+    entries.clear();
+    selectorIndex = 0;
+    updateRequired = true;
+
+    fetchFeed(currentPath);
+  }
+}
+
+void OpdsBookBrowserActivity::downloadBook(const OpdsEntry& book) {
+  state = BrowserState::DOWNLOADING;
+  statusMessage = book.title;
+  downloadProgress = 0;
+  downloadTotal = 0;
+  updateRequired = true;
+
+  // Build full download URL
+  std::string downloadUrl = UrlUtils::buildUrl(SETTINGS.opdsServerUrl, book.href);
+
+  // Create sanitized filename: "Title - Author.epub" or just "Title.epub" if no author
+  std::string baseName = book.title;
+  if (!book.author.empty()) {
+    baseName += " - " + book.author;
+  }
+  std::string filename = "/" + StringUtils::sanitizeFilename(baseName) + ".epub";
+
+  Serial.printf("[%lu] [OPDS] Downloading: %s -> %s\n", millis(), downloadUrl.c_str(), filename.c_str());
+
+  const auto result =
+      HttpDownloader::downloadToFile(downloadUrl, filename, [this](const size_t downloaded, const size_t total) {
+        downloadProgress = downloaded;
+        downloadTotal = total;
+        updateRequired = true;
+      });
+
+  if (result == HttpDownloader::OK) {
+    Serial.printf("[%lu] [OPDS] Download complete: %s\n", millis(), filename.c_str());
+    state = BrowserState::BROWSING;
+    updateRequired = true;
+  } else {
+    state = BrowserState::ERROR;
+    errorMessage = "Download failed";
+    updateRequired = true;
+  }
+}
+
+void OpdsBookBrowserActivity::checkAndConnectWifi() {
+  // Already connected?
+  if (WiFi.status() == WL_CONNECTED) {
+    state = BrowserState::LOADING;
+    statusMessage = "Loading...";
+    updateRequired = true;
+    fetchFeed(currentPath);
+    return;
+  }
+
+  // Try to connect using saved credentials
+  statusMessage = "Connecting to WiFi...";
+  updateRequired = true;
+
+  WIFI_STORE.loadFromFile();
+  const auto& credentials = WIFI_STORE.getCredentials();
+  if (credentials.empty()) {
+    state = BrowserState::ERROR;
+    errorMessage = "No WiFi credentials saved";
+    updateRequired = true;
+    return;
+  }
+
+  // Use the first saved credential
+  const auto& cred = credentials[0];
+  WiFi.mode(WIFI_STA);
+  WiFi.begin(cred.ssid.c_str(), cred.password.c_str());
+
+  // Wait for connection with timeout
+  constexpr int WIFI_TIMEOUT_MS = 10000;
+  const unsigned long startTime = millis();
+  while (WiFi.status() != WL_CONNECTED && millis() - startTime < WIFI_TIMEOUT_MS) {
+    vTaskDelay(100 / portTICK_PERIOD_MS);
+  }
+
+  if (WiFi.status() == WL_CONNECTED) {
+    Serial.printf("[%lu] [OPDS] WiFi connected: %s\n", millis(), WiFi.localIP().toString().c_str());
+    state = BrowserState::LOADING;
+    statusMessage = "Loading...";
+    updateRequired = true;
+    fetchFeed(currentPath);
+  } else {
+    state = BrowserState::ERROR;
+    errorMessage = "WiFi connection failed";
+    updateRequired = true;
+  }
+}

src/activities/browser/OpdsBookBrowserActivity.h

@@ -0,0 +1,61 @@
+#pragma once
+#include <OpdsParser.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+/**
+ * Activity for browsing and downloading books from an OPDS server.
+ * Supports navigation through catalog hierarchy and downloading EPUBs.
+ */
+class OpdsBookBrowserActivity final : public Activity {
+ public:
+  enum class BrowserState {
+    CHECK_WIFI,   // Checking WiFi connection
+    LOADING,      // Fetching OPDS feed
+    BROWSING,     // Displaying entries (navigation or books)
+    DOWNLOADING,  // Downloading selected EPUB
+    ERROR         // Error state with message
+  };
+
+  explicit OpdsBookBrowserActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                   const std::function<void()>& onGoHome)
+      : Activity("OpdsBookBrowser", renderer, mappedInput), onGoHome(onGoHome) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  bool updateRequired = false;
+
+  BrowserState state = BrowserState::LOADING;
+  std::vector<OpdsEntry> entries;
+  std::vector<std::string> navigationHistory;  // Stack of previous feed paths for back navigation
+  std::string currentPath;                     // Current feed path being displayed
+  int selectorIndex = 0;
+  std::string errorMessage;
+  std::string statusMessage;
+  size_t downloadProgress = 0;
+  size_t downloadTotal = 0;
+
+  const std::function<void()> onGoHome;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+
+  void checkAndConnectWifi();
+  void fetchFeed(const std::string& path);
+  void navigateToEntry(const OpdsEntry& entry);
+  void navigateBack();
+  void downloadBook(const OpdsEntry& book);
+};

src/activities/home/HomeActivity.cpp

@@ -4,6 +4,10 @@
 #include <GfxRenderer.h>
 #include <SDCardManager.h>
 
+#include <cstring>
+#include <vector>
+
+#include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "MappedInputManager.h"
 #include "ScreenComponents.h"
@@ -14,7 +18,12 @@ void HomeActivity::taskTrampoline(void* param) {
   self->displayTaskLoop();
 }
 
-int HomeActivity::getMenuItemCount() const { return hasContinueReading ? 4 : 3; }
+int HomeActivity::getMenuItemCount() const {
+  int count = 3;  // Browse files, File transfer, Settings
+  if (hasContinueReading) count++;
+  if (hasOpdsUrl) count++;
+  return count;
+}
 
 void HomeActivity::onEnter() {
   Activity::onEnter();
@@ -24,6 +33,9 @@ void HomeActivity::onEnter() {
   // Check if we have a book to continue reading
   hasContinueReading = !APP_STATE.openEpubPath.empty() && SdMan.exists(APP_STATE.openEpubPath.c_str());
 
+  // Check if OPDS browser URL is configured
+  hasOpdsUrl = strlen(SETTINGS.opdsServerUrl) > 0;
+
   if (hasContinueReading) {
     // Extract filename from path for display
     lastBookTitle = APP_STATE.openEpubPath;
@@ -86,26 +98,24 @@ void HomeActivity::loop() {
   const int menuCount = getMenuItemCount();
 
   if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
-    if (hasContinueReading) {
+    // Calculate dynamic indices based on which options are available
-      // Menu: Continue Reading, Browse, File transfer, Settings
+    int idx = 0;
-      if (selectorIndex == 0) {
+    const int continueIdx = hasContinueReading ? idx++ : -1;
-        onContinueReading();
+    const int browseFilesIdx = idx++;
-      } else if (selectorIndex == 1) {
+    const int opdsLibraryIdx = hasOpdsUrl ? idx++ : -1;
-        onReaderOpen();
+    const int fileTransferIdx = idx++;
-      } else if (selectorIndex == 2) {
+    const int settingsIdx = idx;
-        onFileTransferOpen();
+
-      } else if (selectorIndex == 3) {
+    if (selectorIndex == continueIdx) {
-        onSettingsOpen();
+      onContinueReading();
-      }
+    } else if (selectorIndex == browseFilesIdx) {
-    } else {
+      onReaderOpen();
-      // Menu: Browse, File transfer, Settings
+    } else if (selectorIndex == opdsLibraryIdx) {
-      if (selectorIndex == 0) {
+      onOpdsBrowserOpen();
-        onReaderOpen();
+    } else if (selectorIndex == fileTransferIdx) {
-      } else if (selectorIndex == 1) {
+      onFileTransferOpen();
-        onFileTransferOpen();
+    } else if (selectorIndex == settingsIdx) {
-      } else if (selectorIndex == 2) {
+      onSettingsOpen();
-        onSettingsOpen();
-      }
     }
   } else if (prevPressed) {
     selectorIndex = (selectorIndex + menuCount - 1) % menuCount;
@@ -277,24 +287,31 @@ void HomeActivity::render() const {
     renderer.drawCenteredText(UI_10_FONT_ID, y + renderer.getLineHeight(UI_12_FONT_ID), "Start reading below");
   }
 
-  // --- Bottom menu tiles (indices 1-3) ---
+  // --- Bottom menu tiles ---
-  const int menuTileWidth = pageWidth - 2 * margin;
+  // Build menu items dynamically
-  constexpr int menuTileHeight = 50;
+  std::vector<const char*> menuItems = {"Browse Files", "File Transfer", "Settings"};
-  constexpr int menuSpacing = 10;
+  if (hasOpdsUrl) {
-  constexpr int totalMenuHeight = 3 * menuTileHeight + 2 * menuSpacing;
+    // Insert Calibre Library after Browse Files
+    menuItems.insert(menuItems.begin() + 1, "Calibre Library");
+  }
 
-  int menuStartY = bookY + bookHeight + 20;
+  const int menuTileWidth = pageWidth - 2 * margin;
+  constexpr int menuTileHeight = 45;
+  constexpr int menuSpacing = 8;
+  const int totalMenuHeight =
+      static_cast<int>(menuItems.size()) * menuTileHeight + (static_cast<int>(menuItems.size()) - 1) * menuSpacing;
+
+  int menuStartY = bookY + bookHeight + 15;
   // Ensure we don't collide with the bottom button legend
   const int maxMenuStartY = pageHeight - bottomMargin - totalMenuHeight - margin;
   if (menuStartY > maxMenuStartY) {
     menuStartY = maxMenuStartY;
   }
 
-  for (int i = 0; i < 3; ++i) {
+  for (size_t i = 0; i < menuItems.size(); ++i) {
-    constexpr const char* items[3] = {"Browse files", "File transfer", "Settings"};
+    const int overallIndex = static_cast<int>(i) + (hasContinueReading ? 1 : 0);
-    const int overallIndex = i + (getMenuItemCount() - 3);
     constexpr int tileX = margin;
-    const int tileY = menuStartY + i * (menuTileHeight + menuSpacing);
+    const int tileY = menuStartY + static_cast<int>(i) * (menuTileHeight + menuSpacing);
     const bool selected = selectorIndex == overallIndex;
 
     if (selected) {
@@ -303,7 +320,7 @@ void HomeActivity::render() const {
       renderer.drawRect(tileX, tileY, menuTileWidth, menuTileHeight);
     }
 
-    const char* label = items[i];
+    const char* label = menuItems[i];
     const int textWidth = renderer.getTextWidth(UI_10_FONT_ID, label);
     const int textX = tileX + (menuTileWidth - textWidth) / 2;
     const int lineHeight = renderer.getLineHeight(UI_10_FONT_ID);

src/activities/home/HomeActivity.h

@@ -13,12 +13,14 @@ class HomeActivity final : public Activity {
   int selectorIndex = 0;
   bool updateRequired = false;
   bool hasContinueReading = false;
+  bool hasOpdsUrl = false;
   std::string lastBookTitle;
   std::string lastBookAuthor;
   const std::function<void()> onContinueReading;
   const std::function<void()> onReaderOpen;
   const std::function<void()> onSettingsOpen;
   const std::function<void()> onFileTransferOpen;
+  const std::function<void()> onOpdsBrowserOpen;
 
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
@@ -28,12 +30,14 @@ class HomeActivity final : public Activity {
  public:
   explicit HomeActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
                         const std::function<void()>& onContinueReading, const std::function<void()>& onReaderOpen,
-                        const std::function<void()>& onSettingsOpen, const std::function<void()>& onFileTransferOpen)
+                        const std::function<void()>& onSettingsOpen, const std::function<void()>& onFileTransferOpen,
+                        const std::function<void()>& onOpdsBrowserOpen)
       : Activity("Home", renderer, mappedInput),
         onContinueReading(onContinueReading),
         onReaderOpen(onReaderOpen),
         onSettingsOpen(onSettingsOpen),
-        onFileTransferOpen(onFileTransferOpen) {}
+        onFileTransferOpen(onFileTransferOpen),
+        onOpdsBrowserOpen(onOpdsBrowserOpen) {}
   void onEnter() override;
   void onExit() override;
   void loop() override;

src/activities/network/CalibreWirelessActivity.cpp

@@ -0,0 +1,756 @@
+#include "CalibreWirelessActivity.h"
+
+#include <GfxRenderer.h>
+#include <HardwareSerial.h>
+#include <SDCardManager.h>
+#include <WiFi.h>
+
+#include <cstring>
+
+#include "MappedInputManager.h"
+#include "ScreenComponents.h"
+#include "fontIds.h"
+#include "util/StringUtils.h"
+
+namespace {
+constexpr uint16_t UDP_PORTS[] = {54982, 48123, 39001, 44044, 59678};
+constexpr uint16_t LOCAL_UDP_PORT = 8134;  // Port to receive responses
+}  // namespace
+
+void CalibreWirelessActivity::displayTaskTrampoline(void* param) {
+  auto* self = static_cast<CalibreWirelessActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void CalibreWirelessActivity::networkTaskTrampoline(void* param) {
+  auto* self = static_cast<CalibreWirelessActivity*>(param);
+  self->networkTaskLoop();
+}
+
+void CalibreWirelessActivity::onEnter() {
+  Activity::onEnter();
+
+  renderingMutex = xSemaphoreCreateMutex();
+  stateMutex = xSemaphoreCreateMutex();
+
+  state = WirelessState::DISCOVERING;
+  statusMessage = "Discovering Calibre...";
+  errorMessage.clear();
+  calibreHostname.clear();
+  calibreHost.clear();
+  calibrePort = 0;
+  calibreAltPort = 0;
+  currentFilename.clear();
+  currentFileSize = 0;
+  bytesReceived = 0;
+  inBinaryMode = false;
+  recvBuffer.clear();
+
+  updateRequired = true;
+
+  // Start UDP listener for Calibre responses
+  udp.begin(LOCAL_UDP_PORT);
+
+  // Create display task
+  xTaskCreate(&CalibreWirelessActivity::displayTaskTrampoline, "CalDisplayTask", 2048, this, 1, &displayTaskHandle);
+
+  // Create network task with larger stack for JSON parsing
+  xTaskCreate(&CalibreWirelessActivity::networkTaskTrampoline, "CalNetworkTask", 12288, this, 2, &networkTaskHandle);
+}
+
+void CalibreWirelessActivity::onExit() {
+  Activity::onExit();
+
+  // Turn off WiFi when exiting
+  WiFi.mode(WIFI_OFF);
+
+  // Stop UDP listening
+  udp.stop();
+
+  // Close TCP client if connected
+  if (tcpClient.connected()) {
+    tcpClient.stop();
+  }
+
+  // Close any open file
+  if (currentFile) {
+    currentFile.close();
+  }
+
+  // Acquire stateMutex before deleting network task to avoid race condition
+  xSemaphoreTake(stateMutex, portMAX_DELAY);
+  if (networkTaskHandle) {
+    vTaskDelete(networkTaskHandle);
+    networkTaskHandle = nullptr;
+  }
+  xSemaphoreGive(stateMutex);
+
+  // Acquire renderingMutex before deleting display task
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+
+  vSemaphoreDelete(stateMutex);
+  stateMutex = nullptr;
+}
+
+void CalibreWirelessActivity::loop() {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
+    onComplete();
+    return;
+  }
+}
+
+void CalibreWirelessActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(50 / portTICK_PERIOD_MS);
+  }
+}
+
+void CalibreWirelessActivity::networkTaskLoop() {
+  while (true) {
+    xSemaphoreTake(stateMutex, portMAX_DELAY);
+    const auto currentState = state;
+    xSemaphoreGive(stateMutex);
+
+    switch (currentState) {
+      case WirelessState::DISCOVERING:
+        listenForDiscovery();
+        break;
+
+      case WirelessState::CONNECTING:
+      case WirelessState::WAITING:
+      case WirelessState::RECEIVING:
+        handleTcpClient();
+        break;
+
+      case WirelessState::COMPLETE:
+      case WirelessState::DISCONNECTED:
+      case WirelessState::ERROR:
+        // Just wait, user will exit
+        vTaskDelay(100 / portTICK_PERIOD_MS);
+        break;
+    }
+
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void CalibreWirelessActivity::listenForDiscovery() {
+  // Broadcast "hello" on all UDP discovery ports to find Calibre
+  for (const uint16_t port : UDP_PORTS) {
+    udp.beginPacket("255.255.255.255", port);
+    udp.write(reinterpret_cast<const uint8_t*>("hello"), 5);
+    udp.endPacket();
+  }
+
+  // Wait for Calibre's response
+  vTaskDelay(500 / portTICK_PERIOD_MS);
+
+  // Check for response
+  const int packetSize = udp.parsePacket();
+  if (packetSize > 0) {
+    char buffer[256];
+    const int len = udp.read(buffer, sizeof(buffer) - 1);
+    if (len > 0) {
+      buffer[len] = '\0';
+
+      // Parse Calibre's response format:
+      // "calibre wireless device client (on hostname);port,content_server_port"
+      // or just the hostname and port info
+      std::string response(buffer);
+
+      // Try to extract host and port
+      // Format: "calibre wireless device client (on HOSTNAME);PORT,..."
+      size_t onPos = response.find("(on ");
+      size_t closePos = response.find(')');
+      size_t semiPos = response.find(';');
+      size_t commaPos = response.find(',', semiPos);
+
+      if (semiPos != std::string::npos) {
+        // Get ports after semicolon (format: "port1,port2")
+        std::string portStr;
+        if (commaPos != std::string::npos && commaPos > semiPos) {
+          portStr = response.substr(semiPos + 1, commaPos - semiPos - 1);
+          // Get alternative port after comma
+          std::string altPortStr = response.substr(commaPos + 1);
+          // Trim whitespace and non-digits from alt port
+          size_t altEnd = 0;
+          while (altEnd < altPortStr.size() && altPortStr[altEnd] >= '0' && altPortStr[altEnd] <= '9') {
+            altEnd++;
+          }
+          if (altEnd > 0) {
+            calibreAltPort = static_cast<uint16_t>(std::stoi(altPortStr.substr(0, altEnd)));
+          }
+        } else {
+          portStr = response.substr(semiPos + 1);
+        }
+
+        // Trim whitespace from main port
+        while (!portStr.empty() && (portStr[0] == ' ' || portStr[0] == '\t')) {
+          portStr = portStr.substr(1);
+        }
+
+        if (!portStr.empty()) {
+          calibrePort = static_cast<uint16_t>(std::stoi(portStr));
+        }
+
+        // Get hostname if present, otherwise use sender IP
+        if (onPos != std::string::npos && closePos != std::string::npos && closePos > onPos + 4) {
+          calibreHostname = response.substr(onPos + 4, closePos - onPos - 4);
+        }
+      }
+
+      // Use the sender's IP as the host to connect to
+      calibreHost = udp.remoteIP().toString().c_str();
+      if (calibreHostname.empty()) {
+        calibreHostname = calibreHost;
+      }
+
+      if (calibrePort > 0) {
+        // Connect to Calibre's TCP server - try main port first, then alt port
+        setState(WirelessState::CONNECTING);
+        setStatus("Connecting to " + calibreHostname + "...");
+
+        // Small delay before connecting
+        vTaskDelay(100 / portTICK_PERIOD_MS);
+
+        bool connected = false;
+
+        // Try main port first
+        if (tcpClient.connect(calibreHost.c_str(), calibrePort, 5000)) {
+          connected = true;
+        }
+
+        // Try alternative port if main failed
+        if (!connected && calibreAltPort > 0) {
+          vTaskDelay(200 / portTICK_PERIOD_MS);
+          if (tcpClient.connect(calibreHost.c_str(), calibreAltPort, 5000)) {
+            connected = true;
+          }
+        }
+
+        if (connected) {
+          setState(WirelessState::WAITING);
+          setStatus("Connected to " + calibreHostname + "\nWaiting for commands...");
+        } else {
+          // Don't set error yet, keep trying discovery
+          setState(WirelessState::DISCOVERING);
+          setStatus("Discovering Calibre...\n(Connection failed, retrying)");
+          calibrePort = 0;
+          calibreAltPort = 0;
+        }
+      }
+    }
+  }
+}
+
+void CalibreWirelessActivity::handleTcpClient() {
+  if (!tcpClient.connected()) {
+    setState(WirelessState::DISCONNECTED);
+    setStatus("Calibre disconnected");
+    return;
+  }
+
+  if (inBinaryMode) {
+    receiveBinaryData();
+    return;
+  }
+
+  std::string message;
+  if (readJsonMessage(message)) {
+    // Parse opcode from JSON array format: [opcode, {...}]
+    // Find the opcode (first number after '[')
+    size_t start = message.find('[');
+    if (start != std::string::npos) {
+      start++;
+      size_t end = message.find(',', start);
+      if (end != std::string::npos) {
+        const int opcodeInt = std::stoi(message.substr(start, end - start));
+        if (opcodeInt < 0 || opcodeInt >= OpCode::ERROR) {
+          Serial.printf("[%lu] [CAL] Invalid opcode: %d\n", millis(), opcodeInt);
+          sendJsonResponse(OpCode::OK, "{}");
+          return;
+        }
+        const auto opcode = static_cast<OpCode>(opcodeInt);
+
+        // Extract data object (everything after the comma until the last ']')
+        size_t dataStart = end + 1;
+        size_t dataEnd = message.rfind(']');
+        std::string data = "";
+        if (dataEnd != std::string::npos && dataEnd > dataStart) {
+          data = message.substr(dataStart, dataEnd - dataStart);
+        }
+
+        handleCommand(opcode, data);
+      }
+    }
+  }
+}
+
+bool CalibreWirelessActivity::readJsonMessage(std::string& message) {
+  // Read available data into buffer
+  int available = tcpClient.available();
+  if (available > 0) {
+    // Limit buffer growth to prevent memory issues
+    if (recvBuffer.size() > 100000) {
+      recvBuffer.clear();
+      return false;
+    }
+    // Read in chunks
+    char buf[1024];
+    while (available > 0) {
+      int toRead = std::min(available, static_cast<int>(sizeof(buf)));
+      int bytesRead = tcpClient.read(reinterpret_cast<uint8_t*>(buf), toRead);
+      if (bytesRead > 0) {
+        recvBuffer.append(buf, bytesRead);
+        available -= bytesRead;
+      } else {
+        break;
+      }
+    }
+  }
+
+  if (recvBuffer.empty()) {
+    return false;
+  }
+
+  // Find '[' which marks the start of JSON
+  size_t bracketPos = recvBuffer.find('[');
+  if (bracketPos == std::string::npos) {
+    // No '[' found - if buffer is getting large, something is wrong
+    if (recvBuffer.size() > 1000) {
+      recvBuffer.clear();
+    }
+    return false;
+  }
+
+  // Try to extract length from digits before '['
+  // Calibre ALWAYS sends a length prefix, so if it's not valid digits, it's garbage
+  size_t msgLen = 0;
+  bool validPrefix = false;
+
+  if (bracketPos > 0 && bracketPos <= 12) {
+    // Check if prefix is all digits
+    bool allDigits = true;
+    for (size_t i = 0; i < bracketPos; i++) {
+      char c = recvBuffer[i];
+      if (c < '0' || c > '9') {
+        allDigits = false;
+        break;
+      }
+    }
+    if (allDigits) {
+      msgLen = std::stoul(recvBuffer.substr(0, bracketPos));
+      validPrefix = true;
+    }
+  }
+
+  if (!validPrefix) {
+    // Not a valid length prefix - discard everything up to '[' and treat '[' as start
+    if (bracketPos > 0) {
+      recvBuffer = recvBuffer.substr(bracketPos);
+    }
+    // Without length prefix, we can't reliably parse - wait for more data
+    // that hopefully starts with a proper length prefix
+    return false;
+  }
+
+  // Sanity check the message length
+  if (msgLen > 1000000) {
+    recvBuffer = recvBuffer.substr(bracketPos + 1);  // Skip past this '[' and try again
+    return false;
+  }
+
+  // Check if we have the complete message
+  size_t totalNeeded = bracketPos + msgLen;
+  if (recvBuffer.size() < totalNeeded) {
+    // Not enough data yet - wait for more
+    return false;
+  }
+
+  // Extract the message
+  message = recvBuffer.substr(bracketPos, msgLen);
+
+  // Keep the rest in buffer (may contain binary data or next message)
+  if (recvBuffer.size() > totalNeeded) {
+    recvBuffer = recvBuffer.substr(totalNeeded);
+  } else {
+    recvBuffer.clear();
+  }
+
+  return true;
+}
+
+void CalibreWirelessActivity::sendJsonResponse(const OpCode opcode, const std::string& data) {
+  // Format: length + [opcode, {data}]
+  std::string json = "[" + std::to_string(opcode) + "," + data + "]";
+  const std::string lengthPrefix = std::to_string(json.length());
+  json.insert(0, lengthPrefix);
+
+  tcpClient.write(reinterpret_cast<const uint8_t*>(json.c_str()), json.length());
+  tcpClient.flush();
+}
+
+void CalibreWirelessActivity::handleCommand(const OpCode opcode, const std::string& data) {
+  switch (opcode) {
+    case OpCode::GET_INITIALIZATION_INFO:
+      handleGetInitializationInfo(data);
+      break;
+    case OpCode::GET_DEVICE_INFORMATION:
+      handleGetDeviceInformation();
+      break;
+    case OpCode::FREE_SPACE:
+      handleFreeSpace();
+      break;
+    case OpCode::GET_BOOK_COUNT:
+      handleGetBookCount();
+      break;
+    case OpCode::SEND_BOOK:
+      handleSendBook(data);
+      break;
+    case OpCode::SEND_BOOK_METADATA:
+      handleSendBookMetadata(data);
+      break;
+    case OpCode::DISPLAY_MESSAGE:
+      handleDisplayMessage(data);
+      break;
+    case OpCode::NOOP:
+      handleNoop(data);
+      break;
+    case OpCode::SET_CALIBRE_DEVICE_INFO:
+    case OpCode::SET_CALIBRE_DEVICE_NAME:
+      // These set metadata about the connected Calibre instance.
+      // We don't need this info, just acknowledge receipt.
+      sendJsonResponse(OpCode::OK, "{}");
+      break;
+    case OpCode::SET_LIBRARY_INFO:
+      // Library metadata (name, UUID) - not needed for receiving books
+      sendJsonResponse(OpCode::OK, "{}");
+      break;
+    case OpCode::SEND_BOOKLISTS:
+      // Calibre asking us to send our book list. We report 0 books in
+      // handleGetBookCount, so this is effectively a no-op.
+      sendJsonResponse(OpCode::OK, "{}");
+      break;
+    case OpCode::TOTAL_SPACE:
+      handleFreeSpace();
+      break;
+    default:
+      Serial.printf("[%lu] [CAL] Unknown opcode: %d\n", millis(), opcode);
+      sendJsonResponse(OpCode::OK, "{}");
+      break;
+  }
+}
+
+void CalibreWirelessActivity::handleGetInitializationInfo(const std::string& data) {
+  setState(WirelessState::WAITING);
+  setStatus("Connected to " + calibreHostname +
+            "\nWaiting for transfer...\n\nIf transfer fails, enable\n'Ignore free space' in Calibre's\nSmartDevice "
+            "plugin settings.");
+
+  // Build response with device capabilities
+  // Format must match what Calibre expects from a smart device
+  std::string response = "{";
+  response += "\"appName\":\"CrossPoint\",";
+  response += "\"acceptedExtensions\":[\"epub\"],";
+  response += "\"cacheUsesLpaths\":true,";
+  response += "\"canAcceptLibraryInfo\":true,";
+  response += "\"canDeleteMultipleBooks\":true,";
+  response += "\"canReceiveBookBinary\":true,";
+  response += "\"canSendOkToSendbook\":true,";
+  response += "\"canStreamBooks\":true,";
+  response += "\"canStreamMetadata\":true,";
+  response += "\"canUseCachedMetadata\":true,";
+  // ccVersionNumber: Calibre Companion protocol version. 212 matches CC 5.4.20+.
+  // Using a known version ensures compatibility with Calibre's feature detection.
+  response += "\"ccVersionNumber\":212,";
+  // coverHeight: Max cover image height. We don't process covers, so this is informational only.
+  response += "\"coverHeight\":800,";
+  response += "\"deviceKind\":\"CrossPoint\",";
+  response += "\"deviceName\":\"CrossPoint\",";
+  response += "\"extensionPathLengths\":{\"epub\":37},";
+  response += "\"maxBookContentPacketLen\":4096,";
+  response += "\"passwordHash\":\"\",";
+  response += "\"useUuidFileNames\":false,";
+  response += "\"versionOK\":true";
+  response += "}";
+
+  sendJsonResponse(OpCode::OK, response);
+}
+
+void CalibreWirelessActivity::handleGetDeviceInformation() {
+  std::string response = "{";
+  response += "\"device_info\":{";
+  response += "\"device_store_uuid\":\"" + getDeviceUuid() + "\",";
+  response += "\"device_name\":\"CrossPoint Reader\",";
+  response += "\"device_version\":\"" CROSSPOINT_VERSION "\"";
+  response += "},";
+  response += "\"version\":1,";
+  response += "\"device_version\":\"" CROSSPOINT_VERSION "\"";
+  response += "}";
+
+  sendJsonResponse(OpCode::OK, response);
+}
+
+void CalibreWirelessActivity::handleFreeSpace() {
+  // TODO: Report actual SD card free space instead of hardcoded value
+  // Report 10GB free space for now
+  sendJsonResponse(OpCode::OK, "{\"free_space_on_device\":10737418240}");
+}
+
+void CalibreWirelessActivity::handleGetBookCount() {
+  // We report 0 books - Calibre will send books without checking for duplicates
+  std::string response = "{\"count\":0,\"willStream\":true,\"willScan\":false}";
+  sendJsonResponse(OpCode::OK, response);
+}
+
+void CalibreWirelessActivity::handleSendBook(const std::string& data) {
+  // Manually extract lpath and length from SEND_BOOK data
+  // Full JSON parsing crashes on large metadata, so we just extract what we need
+
+  // Extract "lpath" field - format: "lpath": "value"
+  std::string lpath;
+  size_t lpathPos = data.find("\"lpath\"");
+  if (lpathPos != std::string::npos) {
+    size_t colonPos = data.find(':', lpathPos + 7);
+    if (colonPos != std::string::npos) {
+      size_t quoteStart = data.find('"', colonPos + 1);
+      if (quoteStart != std::string::npos) {
+        size_t quoteEnd = data.find('"', quoteStart + 1);
+        if (quoteEnd != std::string::npos) {
+          lpath = data.substr(quoteStart + 1, quoteEnd - quoteStart - 1);
+        }
+      }
+    }
+  }
+
+  // Extract top-level "length" field - must track depth to skip nested objects
+  // The metadata contains nested "length" fields (e.g., cover image length)
+  size_t length = 0;
+  int depth = 0;
+  for (size_t i = 0; i < data.size(); i++) {
+    char c = data[i];
+    if (c == '{' || c == '[') {
+      depth++;
+    } else if (c == '}' || c == ']') {
+      depth--;
+    } else if (depth == 1 && c == '"') {
+      // At top level, check if this is "length"
+      if (i + 9 < data.size() && data.substr(i, 8) == "\"length\"") {
+        // Found top-level "length" - extract the number after ':'
+        size_t colonPos = data.find(':', i + 8);
+        if (colonPos != std::string::npos) {
+          size_t numStart = colonPos + 1;
+          while (numStart < data.size() && (data[numStart] == ' ' || data[numStart] == '\t')) {
+            numStart++;
+          }
+          size_t numEnd = numStart;
+          while (numEnd < data.size() && data[numEnd] >= '0' && data[numEnd] <= '9') {
+            numEnd++;
+          }
+          if (numEnd > numStart) {
+            length = std::stoul(data.substr(numStart, numEnd - numStart));
+            break;
+          }
+        }
+      }
+    }
+  }
+
+  if (lpath.empty() || length == 0) {
+    sendJsonResponse(OpCode::ERROR, "{\"message\":\"Invalid book data\"}");
+    return;
+  }
+
+  // Extract filename from lpath
+  std::string filename = lpath;
+  const size_t lastSlash = filename.rfind('/');
+  if (lastSlash != std::string::npos) {
+    filename = filename.substr(lastSlash + 1);
+  }
+
+  // Sanitize and create full path
+  currentFilename = "/" + StringUtils::sanitizeFilename(filename);
+  if (currentFilename.find(".epub") == std::string::npos) {
+    currentFilename += ".epub";
+  }
+  currentFileSize = length;
+  bytesReceived = 0;
+
+  setState(WirelessState::RECEIVING);
+  setStatus("Receiving: " + filename);
+
+  // Open file for writing
+  if (!SdMan.openFileForWrite("CAL", currentFilename.c_str(), currentFile)) {
+    setError("Failed to create file");
+    sendJsonResponse(OpCode::ERROR, "{\"message\":\"Failed to create file\"}");
+    return;
+  }
+
+  // Send OK to start receiving binary data
+  sendJsonResponse(OpCode::OK, "{}");
+
+  // Switch to binary mode
+  inBinaryMode = true;
+  binaryBytesRemaining = length;
+
+  // Check if recvBuffer has leftover data (binary file data that arrived with the JSON)
+  if (!recvBuffer.empty()) {
+    size_t toWrite = std::min(recvBuffer.size(), binaryBytesRemaining);
+    size_t written = currentFile.write(reinterpret_cast<const uint8_t*>(recvBuffer.data()), toWrite);
+    bytesReceived += written;
+    binaryBytesRemaining -= written;
+    recvBuffer = recvBuffer.substr(toWrite);
+    updateRequired = true;
+  }
+}
+
+void CalibreWirelessActivity::handleSendBookMetadata(const std::string& data) {
+  // We receive metadata after the book - just acknowledge
+  sendJsonResponse(OpCode::OK, "{}");
+}
+
+void CalibreWirelessActivity::handleDisplayMessage(const std::string& data) {
+  // Calibre may send messages to display
+  // Check messageKind - 1 means password error
+  if (data.find("\"messageKind\":1") != std::string::npos) {
+    setError("Password required");
+  }
+  sendJsonResponse(OpCode::OK, "{}");
+}
+
+void CalibreWirelessActivity::handleNoop(const std::string& data) {
+  // Check for ejecting flag
+  if (data.find("\"ejecting\":true") != std::string::npos) {
+    setState(WirelessState::DISCONNECTED);
+    setStatus("Calibre disconnected");
+  }
+  sendJsonResponse(OpCode::NOOP, "{}");
+}
+
+void CalibreWirelessActivity::receiveBinaryData() {
+  const int available = tcpClient.available();
+  if (available == 0) {
+    // Check if connection is still alive
+    if (!tcpClient.connected()) {
+      currentFile.close();
+      inBinaryMode = false;
+      setError("Transfer interrupted");
+    }
+    return;
+  }
+
+  uint8_t buffer[1024];
+  const size_t toRead = std::min(sizeof(buffer), binaryBytesRemaining);
+  const size_t bytesRead = tcpClient.read(buffer, toRead);
+
+  if (bytesRead > 0) {
+    currentFile.write(buffer, bytesRead);
+    bytesReceived += bytesRead;
+    binaryBytesRemaining -= bytesRead;
+    updateRequired = true;
+
+    if (binaryBytesRemaining == 0) {
+      // Transfer complete
+      currentFile.flush();
+      currentFile.close();
+      inBinaryMode = false;
+
+      setState(WirelessState::WAITING);
+      setStatus("Received: " + currentFilename + "\nWaiting for more...");
+
+      // Send OK to acknowledge completion
+      sendJsonResponse(OpCode::OK, "{}");
+    }
+  }
+}
+
+void CalibreWirelessActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  const auto pageHeight = renderer.getScreenHeight();
+
+  // Draw header
+  renderer.drawCenteredText(UI_12_FONT_ID, 30, "Calibre Wireless", true, EpdFontFamily::BOLD);
+
+  // Draw IP address
+  const std::string ipAddr = WiFi.localIP().toString().c_str();
+  renderer.drawCenteredText(UI_10_FONT_ID, 60, ("IP: " + ipAddr).c_str());
+
+  // Draw status message
+  int statusY = pageHeight / 2 - 40;
+
+  // Split status message by newlines and draw each line
+  std::string status = statusMessage;
+  size_t pos = 0;
+  while ((pos = status.find('\n')) != std::string::npos) {
+    renderer.drawCenteredText(UI_10_FONT_ID, statusY, status.substr(0, pos).c_str());
+    statusY += 25;
+    status = status.substr(pos + 1);
+  }
+  if (!status.empty()) {
+    renderer.drawCenteredText(UI_10_FONT_ID, statusY, status.c_str());
+    statusY += 25;
+  }
+
+  // Draw progress if receiving
+  if (state == WirelessState::RECEIVING && currentFileSize > 0) {
+    const int barWidth = pageWidth - 100;
+    constexpr int barHeight = 20;
+    constexpr int barX = 50;
+    const int barY = statusY + 20;
+    ScreenComponents::drawProgressBar(renderer, barX, barY, barWidth, barHeight, bytesReceived, currentFileSize);
+  }
+
+  // Draw error if present
+  if (!errorMessage.empty()) {
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight - 120, errorMessage.c_str());
+  }
+
+  // Draw button hints
+  const auto labels = mappedInput.mapLabels("Back", "", "", "");
+  renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  renderer.displayBuffer();
+}
+
+std::string CalibreWirelessActivity::getDeviceUuid() const {
+  // Generate a consistent UUID based on MAC address
+  uint8_t mac[6];
+  WiFi.macAddress(mac);
+
+  char uuid[37];
+  snprintf(uuid, sizeof(uuid), "%02x%02x%02x%02x-%02x%02x-4000-8000-%02x%02x%02x%02x%02x%02x", mac[0], mac[1], mac[2],
+           mac[3], mac[4], mac[5], mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+
+  return std::string(uuid);
+}
+
+void CalibreWirelessActivity::setState(WirelessState newState) {
+  xSemaphoreTake(stateMutex, portMAX_DELAY);
+  state = newState;
+  xSemaphoreGive(stateMutex);
+  updateRequired = true;
+}
+
+void CalibreWirelessActivity::setStatus(const std::string& message) {
+  statusMessage = message;
+  updateRequired = true;
+}
+
+void CalibreWirelessActivity::setError(const std::string& message) {
+  errorMessage = message;
+  setState(WirelessState::ERROR);
+}

src/activities/network/CalibreWirelessActivity.h

@@ -0,0 +1,135 @@
+#pragma once
+#include <SDCardManager.h>
+#include <WiFiClient.h>
+#include <WiFiUdp.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+
+#include "activities/Activity.h"
+
+/**
+ * CalibreWirelessActivity implements Calibre's "wireless device" protocol.
+ * This allows Calibre desktop to send books directly to the device over WiFi.
+ *
+ * Protocol specification sourced from Calibre's smart device driver:
+ * https://github.com/kovidgoyal/calibre/blob/master/src/calibre/devices/smart_device_app/driver.py
+ *
+ * Protocol overview:
+ * 1. Device broadcasts "hello" on UDP ports 54982, 48123, 39001, 44044, 59678
+ * 2. Calibre responds with its TCP server address
+ * 3. Device connects to Calibre's TCP server
+ * 4. Calibre sends JSON commands with length-prefixed messages
+ * 5. Books are transferred as binary data after SEND_BOOK command
+ */
+class CalibreWirelessActivity final : public Activity {
+  // Calibre wireless device states
+  enum class WirelessState {
+    DISCOVERING,   // Listening for Calibre server broadcasts
+    CONNECTING,    // Establishing TCP connection
+    WAITING,       // Connected, waiting for commands
+    RECEIVING,     // Receiving a book file
+    COMPLETE,      // Transfer complete
+    DISCONNECTED,  // Calibre disconnected
+    ERROR          // Connection/transfer error
+  };
+
+  // Calibre protocol opcodes (from calibre/devices/smart_device_app/driver.py)
+  enum OpCode : uint8_t {
+    OK = 0,
+    SET_CALIBRE_DEVICE_INFO = 1,
+    SET_CALIBRE_DEVICE_NAME = 2,
+    GET_DEVICE_INFORMATION = 3,
+    TOTAL_SPACE = 4,
+    FREE_SPACE = 5,
+    GET_BOOK_COUNT = 6,
+    SEND_BOOKLISTS = 7,
+    SEND_BOOK = 8,
+    GET_INITIALIZATION_INFO = 9,
+    BOOK_DONE = 11,
+    NOOP = 12,  // Was incorrectly 18
+    DELETE_BOOK = 13,
+    GET_BOOK_FILE_SEGMENT = 14,
+    GET_BOOK_METADATA = 15,
+    SEND_BOOK_METADATA = 16,
+    DISPLAY_MESSAGE = 17,
+    CALIBRE_BUSY = 18,
+    SET_LIBRARY_INFO = 19,
+    ERROR = 20,
+  };
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  TaskHandle_t networkTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  SemaphoreHandle_t stateMutex = nullptr;
+  bool updateRequired = false;
+
+  WirelessState state = WirelessState::DISCOVERING;
+  const std::function<void()> onComplete;
+
+  // UDP discovery
+  WiFiUDP udp;
+
+  // TCP connection (we connect to Calibre)
+  WiFiClient tcpClient;
+  std::string calibreHost;
+  uint16_t calibrePort = 0;
+  uint16_t calibreAltPort = 0;  // Alternative port (content server)
+  std::string calibreHostname;
+
+  // Transfer state
+  std::string currentFilename;
+  size_t currentFileSize = 0;
+  size_t bytesReceived = 0;
+  std::string statusMessage;
+  std::string errorMessage;
+
+  // Protocol state
+  bool inBinaryMode = false;
+  size_t binaryBytesRemaining = 0;
+  FsFile currentFile;
+  std::string recvBuffer;  // Buffer for incoming data (like KOReader)
+
+  static void displayTaskTrampoline(void* param);
+  static void networkTaskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  [[noreturn]] void networkTaskLoop();
+  void render() const;
+
+  // Network operations
+  void listenForDiscovery();
+  void handleTcpClient();
+  bool readJsonMessage(std::string& message);
+  void sendJsonResponse(OpCode opcode, const std::string& data);
+  void handleCommand(OpCode opcode, const std::string& data);
+  void receiveBinaryData();
+
+  // Protocol handlers
+  void handleGetInitializationInfo(const std::string& data);
+  void handleGetDeviceInformation();
+  void handleFreeSpace();
+  void handleGetBookCount();
+  void handleSendBook(const std::string& data);
+  void handleSendBookMetadata(const std::string& data);
+  void handleDisplayMessage(const std::string& data);
+  void handleNoop(const std::string& data);
+
+  // Utility
+  std::string getDeviceUuid() const;
+  void setState(WirelessState newState);
+  void setStatus(const std::string& message);
+  void setError(const std::string& message);
+
+ public:
+  explicit CalibreWirelessActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                   const std::function<void()>& onComplete)
+      : Activity("CalibreWireless", renderer, mappedInput), onComplete(onComplete) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  bool preventAutoSleep() override { return true; }
+  bool skipLoopDelay() override { return true; }
+};

src/activities/settings/CalibreSettingsActivity.cpp

@@ -0,0 +1,169 @@
+#include "CalibreSettingsActivity.h"
+
+#include <GfxRenderer.h>
+#include <WiFi.h>
+
+#include <cstring>
+
+#include "CrossPointSettings.h"
+#include "MappedInputManager.h"
+#include "activities/network/CalibreWirelessActivity.h"
+#include "activities/network/WifiSelectionActivity.h"
+#include "activities/util/KeyboardEntryActivity.h"
+#include "fontIds.h"
+
+namespace {
+constexpr int MENU_ITEMS = 2;
+const char* menuNames[MENU_ITEMS] = {"Calibre Web URL", "Connect as Wireless Device"};
+}  // namespace
+
+void CalibreSettingsActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<CalibreSettingsActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void CalibreSettingsActivity::onEnter() {
+  ActivityWithSubactivity::onEnter();
+
+  renderingMutex = xSemaphoreCreateMutex();
+  selectedIndex = 0;
+  updateRequired = true;
+
+  xTaskCreate(&CalibreSettingsActivity::taskTrampoline, "CalibreSettingsTask",
+              4096,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void CalibreSettingsActivity::onExit() {
+  ActivityWithSubactivity::onExit();
+
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+}
+
+void CalibreSettingsActivity::loop() {
+  if (subActivity) {
+    subActivity->loop();
+    return;
+  }
+
+  if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
+    onBack();
+    return;
+  }
+
+  if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
+    handleSelection();
+    return;
+  }
+
+  if (mappedInput.wasPressed(MappedInputManager::Button::Up) ||
+      mappedInput.wasPressed(MappedInputManager::Button::Left)) {
+    selectedIndex = (selectedIndex + MENU_ITEMS - 1) % MENU_ITEMS;
+    updateRequired = true;
+  } else if (mappedInput.wasPressed(MappedInputManager::Button::Down) ||
+             mappedInput.wasPressed(MappedInputManager::Button::Right)) {
+    selectedIndex = (selectedIndex + 1) % MENU_ITEMS;
+    updateRequired = true;
+  }
+}
+
+void CalibreSettingsActivity::handleSelection() {
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+
+  if (selectedIndex == 0) {
+    // Calibre Web URL
+    exitActivity();
+    enterNewActivity(new KeyboardEntryActivity(
+        renderer, mappedInput, "Calibre Web URL", SETTINGS.opdsServerUrl, 10,
+        127,    // maxLength
+        false,  // not password
+        [this](const std::string& url) {
+          strncpy(SETTINGS.opdsServerUrl, url.c_str(), sizeof(SETTINGS.opdsServerUrl) - 1);
+          SETTINGS.opdsServerUrl[sizeof(SETTINGS.opdsServerUrl) - 1] = '\0';
+          SETTINGS.saveToFile();
+          exitActivity();
+          updateRequired = true;
+        },
+        [this]() {
+          exitActivity();
+          updateRequired = true;
+        }));
+  } else if (selectedIndex == 1) {
+    // Wireless Device - launch the activity (handles WiFi connection internally)
+    exitActivity();
+    if (WiFi.status() != WL_CONNECTED) {
+      enterNewActivity(new WifiSelectionActivity(renderer, mappedInput, [this](bool connected) {
+        exitActivity();
+        if (connected) {
+          enterNewActivity(new CalibreWirelessActivity(renderer, mappedInput, [this] {
+            exitActivity();
+            updateRequired = true;
+          }));
+        } else {
+          updateRequired = true;
+        }
+      }));
+    } else {
+      enterNewActivity(new CalibreWirelessActivity(renderer, mappedInput, [this] {
+        exitActivity();
+        updateRequired = true;
+      }));
+    }
+  }
+
+  xSemaphoreGive(renderingMutex);
+}
+
+void CalibreSettingsActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired && !subActivity) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void CalibreSettingsActivity::render() {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+
+  // Draw header
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, "Calibre", true, EpdFontFamily::BOLD);
+
+  // Draw selection highlight
+  renderer.fillRect(0, 60 + selectedIndex * 30 - 2, pageWidth - 1, 30);
+
+  // Draw menu items
+  for (int i = 0; i < MENU_ITEMS; i++) {
+    const int settingY = 60 + i * 30;
+    const bool isSelected = (i == selectedIndex);
+
+    renderer.drawText(UI_10_FONT_ID, 20, settingY, menuNames[i], !isSelected);
+
+    // Draw status for URL setting
+    if (i == 0) {
+      const char* status = (strlen(SETTINGS.opdsServerUrl) > 0) ? "[Set]" : "[Not Set]";
+      const auto width = renderer.getTextWidth(UI_10_FONT_ID, status);
+      renderer.drawText(UI_10_FONT_ID, pageWidth - 20 - width, settingY, status, !isSelected);
+    }
+  }
+
+  // Draw button hints
+  const auto labels = mappedInput.mapLabels("« Back", "Select", "", "");
+  renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  renderer.displayBuffer();
+}

src/activities/settings/CalibreSettingsActivity.h

@@ -0,0 +1,36 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+
+#include "activities/ActivityWithSubactivity.h"
+
+/**
+ * Submenu for Calibre settings.
+ * Shows Calibre Web URL and Calibre Wireless Device options.
+ */
+class CalibreSettingsActivity final : public ActivityWithSubactivity {
+ public:
+  explicit CalibreSettingsActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                   const std::function<void()>& onBack)
+      : ActivityWithSubactivity("CalibreSettings", renderer, mappedInput), onBack(onBack) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  bool updateRequired = false;
+
+  int selectedIndex = 0;
+  const std::function<void()> onBack;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render();
+  void handleSelection();
+};

src/activities/settings/SettingsActivity.cpp

@@ -3,6 +3,9 @@
 #include <GfxRenderer.h>
 #include <HardwareSerial.h>
 
+#include <cstring>
+
+#include "CalibreSettingsActivity.h"
 #include "CrossPointSettings.h"
 #include "MappedInputManager.h"
 #include "OtaUpdateActivity.h"
@@ -10,7 +13,7 @@
 
 // Define the static settings list
 namespace {
-constexpr int settingsCount = 16;
+constexpr int settingsCount = 17;
 const SettingInfo settingsList[settingsCount] = {
     // Should match with SLEEP_SCREEN_MODE
     SettingInfo::Enum("Sleep Screen", &CrossPointSettings::sleepScreen, {"Dark", "Light", "Custom", "Cover", "None"}),
@@ -35,6 +38,7 @@ const SettingInfo settingsList[settingsCount] = {
                       {"1 min", "5 min", "10 min", "15 min", "30 min"}),
     SettingInfo::Enum("Refresh Frequency", &CrossPointSettings::refreshFrequency,
                       {"1 page", "5 pages", "10 pages", "15 pages", "30 pages"}),
+    SettingInfo::Action("Calibre Settings"),
     SettingInfo::Action("Check for updates")};
 }  // namespace
 
@@ -132,7 +136,15 @@ void SettingsActivity::toggleCurrentSetting() {
       SETTINGS.*(setting.valuePtr) = currentValue + setting.valueRange.step;
     }
   } else if (setting.type == SettingType::ACTION) {
-    if (std::string(setting.name) == "Check for updates") {
+    if (strcmp(setting.name, "Calibre Settings") == 0) {
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      exitActivity();
+      enterNewActivity(new CalibreSettingsActivity(renderer, mappedInput, [this] {
+        exitActivity();
+        updateRequired = true;
+      }));
+      xSemaphoreGive(renderingMutex);
+    } else if (strcmp(setting.name, "Check for updates") == 0) {
       xSemaphoreTake(renderingMutex, portMAX_DELAY);
       exitActivity();
       enterNewActivity(new OtaUpdateActivity(renderer, mappedInput, [this] {

src/main.cpp

@@ -7,12 +7,15 @@
 #include <SPI.h>
 #include <builtinFonts/all.h>
 
+#include <cstring>
+
 #include "Battery.h"
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "MappedInputManager.h"
 #include "activities/boot_sleep/BootActivity.h"
 #include "activities/boot_sleep/SleepActivity.h"
+#include "activities/browser/OpdsBookBrowserActivity.h"
 #include "activities/home/HomeActivity.h"
 #include "activities/network/CrossPointWebServerActivity.h"
 #include "activities/reader/ReaderActivity.h"
@@ -222,10 +225,15 @@ void onGoToSettings() {
   enterNewActivity(new SettingsActivity(renderer, mappedInputManager, onGoHome));
 }
 
+void onGoToBrowser() {
+  exitActivity();
+  enterNewActivity(new OpdsBookBrowserActivity(renderer, mappedInputManager, onGoHome));
+}
+
 void onGoHome() {
   exitActivity();
   enterNewActivity(new HomeActivity(renderer, mappedInputManager, onContinueReading, onGoToReaderHome, onGoToSettings,
-                                    onGoToFileTransfer));
+                                    onGoToFileTransfer, onGoToBrowser));
 }
 
 void setupDisplayAndFonts() {

src/network/HttpDownloader.cpp

@@ -0,0 +1,128 @@
+#include "HttpDownloader.h"
+
+#include <HTTPClient.h>
+#include <HardwareSerial.h>
+#include <WiFiClientSecure.h>
+
+#include <memory>
+
+bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
+  const std::unique_ptr<WiFiClientSecure> client(new WiFiClientSecure());
+  client->setInsecure();
+  HTTPClient http;
+
+  Serial.printf("[%lu] [HTTP] Fetching: %s\n", millis(), url.c_str());
+
+  http.begin(*client, url.c_str());
+  http.setFollowRedirects(HTTPC_STRICT_FOLLOW_REDIRECTS);
+  http.addHeader("User-Agent", "CrossPoint-ESP32-" CROSSPOINT_VERSION);
+
+  const int httpCode = http.GET();
+  if (httpCode != HTTP_CODE_OK) {
+    Serial.printf("[%lu] [HTTP] Fetch failed: %d\n", millis(), httpCode);
+    http.end();
+    return false;
+  }
+
+  outContent = http.getString().c_str();
+  http.end();
+
+  Serial.printf("[%lu] [HTTP] Fetched %zu bytes\n", millis(), outContent.size());
+  return true;
+}
+
+HttpDownloader::DownloadError HttpDownloader::downloadToFile(const std::string& url, const std::string& destPath,
+                                                             ProgressCallback progress) {
+  const std::unique_ptr<WiFiClientSecure> client(new WiFiClientSecure());
+  client->setInsecure();
+  HTTPClient http;
+
+  Serial.printf("[%lu] [HTTP] Downloading: %s\n", millis(), url.c_str());
+  Serial.printf("[%lu] [HTTP] Destination: %s\n", millis(), destPath.c_str());
+
+  http.begin(*client, url.c_str());
+  http.setFollowRedirects(HTTPC_STRICT_FOLLOW_REDIRECTS);
+  http.addHeader("User-Agent", "CrossPoint-ESP32-" CROSSPOINT_VERSION);
+
+  const int httpCode = http.GET();
+  if (httpCode != HTTP_CODE_OK) {
+    Serial.printf("[%lu] [HTTP] Download failed: %d\n", millis(), httpCode);
+    http.end();
+    return HTTP_ERROR;
+  }
+
+  const size_t contentLength = http.getSize();
+  Serial.printf("[%lu] [HTTP] Content-Length: %zu\n", millis(), contentLength);
+
+  // Remove existing file if present
+  if (SdMan.exists(destPath.c_str())) {
+    SdMan.remove(destPath.c_str());
+  }
+
+  // Open file for writing
+  FsFile file;
+  if (!SdMan.openFileForWrite("HTTP", destPath.c_str(), file)) {
+    Serial.printf("[%lu] [HTTP] Failed to open file for writing\n", millis());
+    http.end();
+    return FILE_ERROR;
+  }
+
+  // Get the stream for chunked reading
+  WiFiClient* stream = http.getStreamPtr();
+  if (!stream) {
+    Serial.printf("[%lu] [HTTP] Failed to get stream\n", millis());
+    file.close();
+    SdMan.remove(destPath.c_str());
+    http.end();
+    return HTTP_ERROR;
+  }
+
+  // Download in chunks
+  uint8_t buffer[DOWNLOAD_CHUNK_SIZE];
+  size_t downloaded = 0;
+  const size_t total = contentLength > 0 ? contentLength : 0;
+
+  while (http.connected() && (contentLength == 0 || downloaded < contentLength)) {
+    const size_t available = stream->available();
+    if (available == 0) {
+      delay(1);
+      continue;
+    }
+
+    const size_t toRead = available < DOWNLOAD_CHUNK_SIZE ? available : DOWNLOAD_CHUNK_SIZE;
+    const size_t bytesRead = stream->readBytes(buffer, toRead);
+
+    if (bytesRead == 0) {
+      break;
+    }
+
+    const size_t written = file.write(buffer, bytesRead);
+    if (written != bytesRead) {
+      Serial.printf("[%lu] [HTTP] Write failed: wrote %zu of %zu bytes\n", millis(), written, bytesRead);
+      file.close();
+      SdMan.remove(destPath.c_str());
+      http.end();
+      return FILE_ERROR;
+    }
+
+    downloaded += bytesRead;
+
+    if (progress && total > 0) {
+      progress(downloaded, total);
+    }
+  }
+
+  file.close();
+  http.end();
+
+  Serial.printf("[%lu] [HTTP] Downloaded %zu bytes\n", millis(), downloaded);
+
+  // Verify download size if known
+  if (contentLength > 0 && downloaded != contentLength) {
+    Serial.printf("[%lu] [HTTP] Size mismatch: got %zu, expected %zu\n", millis(), downloaded, contentLength);
+    SdMan.remove(destPath.c_str());
+    return HTTP_ERROR;
+  }
+
+  return OK;
+}

src/network/HttpDownloader.h

@@ -0,0 +1,42 @@
+#pragma once
+#include <SDCardManager.h>
+
+#include <functional>
+#include <string>
+
+/**
+ * HTTP client utility for fetching content and downloading files.
+ * Wraps WiFiClientSecure and HTTPClient for HTTPS requests.
+ */
+class HttpDownloader {
+ public:
+  using ProgressCallback = std::function<void(size_t downloaded, size_t total)>;
+
+  enum DownloadError {
+    OK = 0,
+    HTTP_ERROR,
+    FILE_ERROR,
+    ABORTED,
+  };
+
+  /**
+   * Fetch text content from a URL.
+   * @param url The URL to fetch
+   * @param outContent The fetched content (output)
+   * @return true if fetch succeeded, false on error
+   */
+  static bool fetchUrl(const std::string& url, std::string& outContent);
+
+  /**
+   * Download a file to the SD card.
+   * @param url The URL to download
+   * @param destPath The destination path on SD card
+   * @param progress Optional progress callback
+   * @return DownloadError indicating success or failure type
+   */
+  static DownloadError downloadToFile(const std::string& url, const std::string& destPath,
+                                      ProgressCallback progress = nullptr);
+
+ private:
+  static constexpr size_t DOWNLOAD_CHUNK_SIZE = 1024;
+};

src/util/StringUtils.cpp

@@ -0,0 +1,36 @@
+#include "StringUtils.h"
+
+namespace StringUtils {
+
+std::string sanitizeFilename(const std::string& name, size_t maxLength) {
+  std::string result;
+  result.reserve(name.size());
+
+  for (char c : name) {
+    // Replace invalid filename characters with underscore
+    if (c == '/' || c == '\\' || c == ':' || c == '*' || c == '?' || c == '"' || c == '<' || c == '>' || c == '|') {
+      result += '_';
+    } else if (c >= 32 && c < 127) {
+      // Keep printable ASCII characters
+      result += c;
+    }
+    // Skip non-printable characters
+  }
+
+  // Trim leading/trailing spaces and dots
+  size_t start = result.find_first_not_of(" .");
+  if (start == std::string::npos) {
+    return "book";  // Fallback if name is all invalid characters
+  }
+  size_t end = result.find_last_not_of(" .");
+  result = result.substr(start, end - start + 1);
+
+  // Limit filename length
+  if (result.length() > maxLength) {
+    result.resize(maxLength);
+  }
+
+  return result.empty() ? "book" : result;
+}
+
+}  // namespace StringUtils

src/util/StringUtils.h

@@ -0,0 +1,13 @@
+#pragma once
+#include <string>
+
+namespace StringUtils {
+
+/**
+ * Sanitize a string for use as a filename.
+ * Replaces invalid characters with underscores, trims spaces/dots,
+ * and limits length to maxLength characters.
+ */
+std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
+
+}  // namespace StringUtils

src/util/UrlUtils.cpp

@@ -0,0 +1,41 @@
+#include "UrlUtils.h"
+
+namespace UrlUtils {
+
+std::string ensureProtocol(const std::string& url) {
+  if (url.find("://") == std::string::npos) {
+    return "http://" + url;
+  }
+  return url;
+}
+
+std::string extractHost(const std::string& url) {
+  const size_t protocolEnd = url.find("://");
+  if (protocolEnd == std::string::npos) {
+    // No protocol, find first slash
+    const size_t firstSlash = url.find('/');
+    return firstSlash == std::string::npos ? url : url.substr(0, firstSlash);
+  }
+  // Find the first slash after the protocol
+  const size_t hostStart = protocolEnd + 3;
+  const size_t pathStart = url.find('/', hostStart);
+  return pathStart == std::string::npos ? url : url.substr(0, pathStart);
+}
+
+std::string buildUrl(const std::string& serverUrl, const std::string& path) {
+  const std::string urlWithProtocol = ensureProtocol(serverUrl);
+  if (path.empty()) {
+    return urlWithProtocol;
+  }
+  if (path[0] == '/') {
+    // Absolute path - use just the host
+    return extractHost(urlWithProtocol) + path;
+  }
+  // Relative path - append to server URL
+  if (urlWithProtocol.back() == '/') {
+    return urlWithProtocol + path;
+  }
+  return urlWithProtocol + "/" + path;
+}
+
+}  // namespace UrlUtils

src/util/UrlUtils.h

@@ -0,0 +1,23 @@
+#pragma once
+#include <string>
+
+namespace UrlUtils {
+
+/**
+ * Prepend http:// if no protocol specified (server will redirect to https if needed)
+ */
+std::string ensureProtocol(const std::string& url);
+
+/**
+ * Extract host with protocol from URL (e.g., "http://example.com" from "http://example.com/path")
+ */
+std::string extractHost(const std::string& url);
+
+/**
+ * Build full URL from server URL and path.
+ * If path starts with /, it's an absolute path from the host root.
+ * Otherwise, it's relative to the server URL.
+ */
+std::string buildUrl(const std::string& serverUrl, const std::string& path);
+
+}  // namespace UrlUtils