Putting two of the same enchanted books in the grindstone (For example, two "Protection I" books) will merge them into 1 book. Any non-similar enchantments, even the same enchant at a different level on each book, will not allow it (as I assume is the intended way)
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Duplicate of MC-148480
Merging books is done with the anvil. The grindstone is for repairing two tools together or disenchanting.