Do unenchanted books not stack with regular books?
Turns out - yes.
I had 2 stacks; one with 7 books, the other with 4. I enchanted one of each, then when I unenchanted them, they joined the 7 book stack. So I had a stack of 8, and a stack of 3.
I then crafted a new book, and it joined the 3 book stack.
After showing Advanced tooltips (F3 + H) one stack had the tag minecraft:book, the other had minecraft:book NBT: 1 tag(s).
At this point I don't know if this is intentional or a bug.
User 54D has pointed out that this has/will be fixed in a later version:
Per MC-148476 this will be fixed in 1.14+