This is the second and (apparently - if the reviews I've read are correct) very overlooked book in the Snow Queen cycle.
It does not take place on Tiamat or really deal much with Tiamat. The main character is BZ, an inspector on Tiamat who loses his honor in his very strict culture, and tries to find it again.
He goes on a mission to rescue his brothers after becoming obsessed with the fact that they're missing, and what happens is a mystery and deep character study, with a generous helping of descent into madness. There's more information about the sybil network, which is the best part of the storyline, in my opinion, which I think warrants reading as long as you know that this story is linked to, but not anything like, Snow Queen.
It's a good story, and a good short novel. Unfortunately it suffers from coming after an epic and huge novel/story, so it feels like a bit of a letdown.