If you need to retrieve a book you've deleted, click on the little blue arrow beneath the library name in the toolbar and you'll see your Recently Deleted books.

You'll see any books that have been deleted within the last two weeks. What's particularly useful is that even if a student has deleted the book, a teacher can still recover it.

Now click on the 3 dot menu and you'll see the option to either Restore or Delete forever. (In Book Creator for iPad, you tap the + icon to restore a book, or tap the Edit button and then permanently delete books here).