What I expected to happen was...:

I disenchanted books and shift+left click to stack them with the rest.

What actually happened was...:
They didn't stack with the rest of the books that were previously in my inventory.

Steps to Reproduce:
1. Get some books
2. Enchant them in the enchantment table (not all of them)
3. Go to grindstone and disenchant books

4. Disenchanted books don't stack with the rest of them