Tara Abernathy, witch in training, is thrown out of school for unspecified offenses. This is a serious punishment when the school is a magical city floating a thousand feet in the air. You could also look at it as a sink-or-splat graduation test. She does not splat, of course, and winds up as assistant to Ms. Kevarian, partner of the magical concern Kelethras, Albrecht, and Ao. Ms. Kevarian is dealing with an unfortunate situation in the city of Alt Coulumb. Ms. Abernathy has one chance to prove herself competent.
The book has a very nifty take on theological engineering. It's rather the fantasy take on Debt, although I assure you there's way more plot and less lecturing than Neptune's Brood had. In fact the magic is all deft, dense, and creepy while never distracting from the story; think P.C. Hodgell if the Kencyr all wore pinstripe business suits. Without being any less scary.
The only criticism I can level is that we run into vampires who don't really add anything to the vampire trope page. (And, to be fair, Mieville did that too.) Anyhow recommended.