What I expected to happen was...:
The grindstone should return a normal book

What actually happened was...:
The grindstone returned an "enchanted book" without enchantments, was not stackable with normal books and could be enchanted like a normal book.

Steps to Reproduce:
1. Get an enchanted book, I only tried with books that had 1 and 2 enchantments.
2. Put it in a grindstone and take the output.
3. The book you now have is an "enchanted book" without enchantments, non stackable with normal books and can be enchanted.