HELP: my books disappeared from ibooks

Hello, everyone! I bought an iPad air recently and decided to transfer my books from my old iPad 3rd gen to my new iPad air. I've been using it for a while and everything worked fine. Today I decided to add a new book manually via iTunes, but in iTunes I couldn't find my old books so I checked ibooks app on iPad air and all my books were not there. After that I checked my iCloud account in settings and found 65 mb of books there but there are not any books in ibooks application. On my old iPad all books were added manually via iTunes(not from a store). Why did they disappear? Please help, what should I do?

Have you been making backups to either the cloud or iTunes? If you have, restore your books from the backup.