You sure about that? Have you had one drop since the latest patch? Neither me, nor any of my alliance mates have seen a rare book drop from 12-9 since the latest patch… It could be they made the drop rate even smaller in the patch and /or we were all unlucky, I guess, but I’d be happy to know if you’ve see one!
The patch notes did say that “skillbooks feature now unlock later the game”. At first I didn’t assume that meant skillbook won’t drop till later in the game, but until I see proof of a rare skillbook from 12-9, I’m starting to believe it Earliest proof I’ve seen is Area 19 so far.