A good question. I'd imagine that the library itself would be known to only a few people, including the mysteeeerious stranger that enlisted Blacksticks, but the area around it would certainly be open to high-ranking people like Yones and other Mordecai.
Perhaps... And this is just an idea... the library could be a safe deposit box for the various magical tomes that the kingdom has come across. Yones, being a Mordecai, could sense the magic that they create and become curious. I'm guessing that he's the kind of guy that only likes secret when he's in on them.
I don't know why the kingdom would be stockpiling the hated magic books in the first place... Perhaps they're the ones that they couldn't figure out how to destroy, and didn't want to be discovered by untrustworthy out-of-towners. Perhaps someone high-up is a magic-liker! I don't have much experience with the Connlaoth herarchy, but there's always some pretty important untrustworthy character.
And as to why no Mordecai has noticed the library before... Well, I'm sure you'll think of something. Maybe he wasn't supposed to be there after all!
Oooh! A plot is in the thickening!
(BTW, I have no idea who bought Blacksticks' services, yet. But he's definitely mysteeeerious!)