Pretty simple this. On the My books screen, tap the 'Inspector' icon and you'll see the delete button at the bottom.
If you have Book Creator 4.1 or above, deleted books will automatically be added to your Recently Deleted bookshelf where they will remain for 2 weeks.
At this point, you might be interested in how to retrieve a deleted book.
If you need to completely delete a book from Book Creator to free up space on your iPad, tap on the 'All Books' link in the top toolbar to bring up your bookshelves. Tap on the Recently Deleted bookshelf and then swipe left on each book to permanently delete it.
Note, you will not be able to retrieve a book once it's permanently deleted.
If you are using the Free version of Book Creator, if you try to delete your book you will be presented with an Upgrade screen. In the free version, you are only allowed to make one book.
If you upgrade, you can create and delete as many books as you like.
Deleting multiple books
In Book Creator 4.1, we added the option to delete multiple books in the bookshelf view.
1. Tap the Grid icon in the top right when on the My Books screen.
2. Tap Select (top left)
3. Tap on any book and you'll see a tick mark in a blue circle has been added.
4. Lastly, tap the Delete link in the top right of the toolbar.
From the blog: