When grinding an item with only curses (binding or vanishing) and a book, it produces a generic "enchanted book".
The correct behavior is to not produce anything at all, as it is actually impossible to remove a curse from an item.

When the item has a mix of curses and enchants, it first produces a book with all valid enchants, and then goes back to the bug (since the object has now only curses). That may be the intended behavior in that case, or maybe the intended behavior is that it is impossible to create books form cursed objects at all, it's a balance choice.