Completely Deleting a book from Apple Books

You can't completely delete purchase. Since you purchased the book (even if it was a free book), Apple is obligated to always provide a way for you to download it. Hence, hiding the books from the Purchased page in the store is the best you can do. But that does have the effect of removing the book from the Books Library, so I would think that's sufficient for most situations.

Are you talking about a book that you downloaded from the Apple Book Store? If so, then you can hide the purchase, which I assume will give you the result you want.

Go to Book Store, then Featured option on the left, then Purchased link on the right. You should see all your purchased books there. Note that even if the book was free, it counts as a purchase. Hover over the book that you want to get rid of and then click on the 'X' that appears.