You can use the Grindstone to remove enchantments from books.
I have a feeling this is a bug because it makes no sense.
This bug is marked as WAI even though we have no confirmation / statement from the devs
Comments 2
Thank you for your report!
However, this issue is Working as Intended.
Grindstones are used to remove enchantments from any item that can have enchantments or can be enchanted in vanilla Minecraft.
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I believe this is working as intended - grindstone is smoothing the pages maybe? 😉