You can easily delete a book you no longer need. This will free up space in your library if you're nearing the limit.

Just click the Books icon beneath your book, and then choose Delete book.

Note that you can only delete your own books, you can't delete other people's unless you are the owner of the library, or have been promoted to co-teacher.

If you need to restore 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. 

Click on this and 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).

To delete multiple books, tap on the Grid view icon in the toolbar, tap Select, then tap on any books you want to to delete. Then choose Delete.