private void build() {
    // Step 0: get database books marked as "existing"
    final FileInfoSet fileInfos = new FileInfoSet(myDatabase);
    final Map<Long, Book> savedBooksByFileId = myDatabase.loadBooks(fileInfos, true);
    final Map<Long, Book> savedBooksByBookId = new HashMap<Long, Book>();
    for (Book b : savedBooksByFileId.values()) {
      savedBooksByBookId.put(b.getId(), b);
    }

    // Step 1: check if files corresponding to "existing" books really exists;
    //         add books to library if yes (and reload book info if needed);
    //         remove from recent/favorites list if no;
    //         collect newly "orphaned" books
    final Set<Book> orphanedBooks = new HashSet<Book>();
    final Set<ZLPhysicalFile> physicalFiles = new HashSet<ZLPhysicalFile>();
    int count = 0;
    for (Book book : savedBooksByFileId.values()) {
      final ZLPhysicalFile file = book.File.getPhysicalFile();
      if (file != null) {
        physicalFiles.add(file);
      }
      if (file != book.File && file != null && file.getPath().endsWith(".epub")) {
        continue;
      }
      if (book.File.exists()) {
        boolean doAdd = true;
        if (file == null) {
          continue;
        }
        if (!fileInfos.check(file, true)) {
          try {
            book.readMetaInfo();
            saveBook(book, false);
          } catch (BookReadingException e) {
            doAdd = false;
          }
          file.setCached(false);
        }
        if (doAdd) {
          // loaded from db
          addBook(book, false);
        }
      } else {
        orphanedBooks.add(book);
      }
    }
    myDatabase.setExistingFlag(orphanedBooks, false);

    // Step 2: collect books from physical files; add new, update already added,
    //         unmark orphaned as existing again, collect newly added
    final Map<Long, Book> orphanedBooksByFileId = myDatabase.loadBooks(fileInfos, false);
    final Set<Book> newBooks = new HashSet<Book>();

    final List<ZLPhysicalFile> physicalFilesList = collectPhysicalFiles(BookDirectories);
    for (ZLPhysicalFile file : physicalFilesList) {
      if (physicalFiles.contains(file)) {
        continue;
      }
      collectBooks(
          file,
          fileInfos,
          savedBooksByFileId,
          orphanedBooksByFileId,
          newBooks,
          !fileInfos.check(file, true));
      file.setCached(false);
    }

    // Step 3: add help file
    try {
      final ZLFile helpFile = BookUtil.getHelpFile();
      Book helpBook = savedBooksByFileId.get(fileInfos.getId(helpFile));
      if (helpBook == null) {
        helpBook = new Book(helpFile);
      }
      saveBook(helpBook, false);
      // saved
      addBook(helpBook, false);
    } catch (BookReadingException e) {
      // that's impossible
      e.printStackTrace();
    }

    // Step 4: save changes into database
    fileInfos.save();

    myDatabase.executeAsTransaction(
        new Runnable() {
          public void run() {
            for (Book book : newBooks) {
              saveBook(book, false);
            }
          }
        });
    myDatabase.setExistingFlag(newBooks, true);
  }