The answer is actually: No, they don't drop anymore.
The preview from the pachinko isn't accurate and hasn't been since the start (the legendary item preview is the easiest to spot errors in, with it's lack of items), so you should just ignore it... But they didn't drop in 1000 10x spins at all, so it's safe to assume the books don't drop anymore (yes, I had money to waste). I wonder if there's a bug that prevents them from dropping for some people.