Please keep the Book Crawler application open and in the foreground on iOS devices when syncing a database to the cloud. Apple will kill applications that process while in the background and this can corrupt your database file. I also recommend that "Background App Refresh" is turned on in Settings for Book Crawler when using cloud support.
If you placed the application into the background when you initially began syncing with the cloud and "lost" your database, please contact me so that I can give you instructions to recover that data.