iBooks on my ipad is using 5 GB of storage, but has deleted all my books. How can I fix this?

I recently had messages appearing on my ipad saying my storage is almost full. When I checked my settings, I noticed that ibooks is using 5.3 GB, and yet when I opened it to access a book I was studying, and all my books were gone. So there are no longer any books in my iBooks, no epubs, no pdfs. And yet ibooks is still eating up over 5 GB of storage. And whenever I free up space from somewhere else, it seems to eat up that storage too. I've spent a couple of evenings trying to restore from backup, but it's saying I have not enough room on my ipad to perform the restore. And I cannot get more room, because iBooks is taking it all up. I also cannot un-install iBooks, because it is now embedded in the iOS software. Super frustrating, and I can't find any other posts like this. Any thoughts?