Keep a copy of your books
Use Settings → Export library backup to save a portable copy to Files or iCloud Drive. Automatic recovery copies are stored on the device and do not survive uninstalling the app or losing the device. Export before deleting the app while troubleshooting.
iCloud
On your devices, sign in to the same Apple Account and enable iCloud for Bookshelf. Changes sync when Apple’s system permits. Pull down on the Bookshelf page to refresh the local view and check iCloud status; this cannot force Apple’s servers to finish a transfer. Contact support if a sync error persists.
Finding older books
Search by title or author, or add a book manually when no ISBN is available. You can enter only a title and add other details later. Copyright-page recognition offers suggestions for your review; edition and printing are separate, and unknown values can stay blank.