Deleting epub book permanently from app / ipad

I use ebooks reader to read epub book that I downloaded from various sites on to my ebook reader app on my ipad.

The problem I have is that I can’t delete any books.

I got to library-edit- select books- delete and confirm - like I’m supposed to.

The book look like they’ve been deleted but when I open the app back up the deleted book reappear.

Where are the books stored on the app or iPad so I can find a different way to delete the books I’ve read as there is now to many books it’s getting confusing.

please help as tried researching but nothing.