async function getZbook(book : Book) : Promise<Database> {
  let file = files[book.file];
  if (!file) {
    file = await localforage.getItem<BookFile>(book.file);
    if (file) {
      if (file.version !== book.file_version) {
        // Update book in background.
        fetchBinary(
          book.url,
          "Updating book " + book.name + "..."
        ).then(data => {
          file.data = data;
          // TODO: notify UI book has been updated
          files[book.file] = file;
          return localforage.setItem(book.file, file);
        });
      }
    }
    else {
      file = {
        version: book.file_version,
        data: await fetchBinary(
          book.url,
          "Downloading book " + book.name + "..."
        )
      };
      await localforage.setItem(book.file, file);
    }
  }

  return new Database(inflate(file.data));
}

async function getCatalog() {
  let language = await getLanguage();
  let catalog = await localforage.getItem<Folder>("catalog");
  if (catalog &&
      platform == await localforage.getItem("platform") &&
      language == await localforage.getItem("language")) {
    updateCatalog(language); // check for update in background
    return catalog; // But return cached value immediately
  }
  return await updateCatalog(language);
}

async function updateCatalog(language) : Promise<Folder> {
  let update = await fetchJson(
    "http://tech.lds.org/glweb?action=catalog.query.modified&languageid=" +
    language + "&platformid=" + platform + "&format=json",
    "Checking for catalog updates..."
  );
  let version = update.version;
  if (version == await localforage.getItem("version")) {
    console.log("No catalog updates.");
    return;
  }
  let result = await fetchJson(
    "http://tech.lds.org/glweb?action=catalog.query&languageid=" +
    language + "&platformid=" + platform + "&format=json",
    "Downloading catalog..."
  );
  await localforage.setItem("catalog", result.catalog);
  await localforage.setItem("version", version);
  await localforage.setItem("language", language);
  await localforage.setItem("platform", platform);
  // TODO: use book.versions API to update any downloaded books.
  return result.catalog;
}

 ).then(data => {
   file.data = data;
   // TODO: notify UI book has been updated
   files[book.file] = file;
   return localforage.setItem(book.file, file);
 });