The first two books of this series were clearly film noir homages. Not only did they follow many of the noir conventions, but for good measure it was established that many of the main characters were fans of classic noir films.
But Odd Girl Out seems to have shifted gears a bit. It starts with another one of the noir tropes, where the pretty girl asks a detective for help, he turns her down, and then she gets murdered. So now he's on the case, needing to redeem his hardheartedness. Plus, she's murdered with his weapon, so that makes him the prime suspect.
But somewhere along the line, the background story of the Modhri and the Spiders starts taking over the tale. By the end of the book, much of the noir feeling is gone, or at least subsumed. Instead, we are set up to expect that the next book will be more of a straightforward battle. Noir is all about atmosphere and style and misdirection, but now it seems this may be replaced with something more direct. The detente between the Modhri and his foes (including Compton and Bayta) appears to be over.
In a sense, that's too bad. I was enjoying Zahn's flirtation with the noir conventions. But it couldn't have gone on forever, for the same reason noir films generally didn't have sequels. You can only carry a noir story just so far -- after that the suspension of disbelief snaps.
I felt that Odd Girl Out started to lose its way as the story progressed. Zahn started caring more about the greater war than about the little skirmish that Compton once more found himself fighting. And the book suffered a bit because of this. Neither fish nor foul, it ended up feeling more like a transition than anything else. Perhaps you could call it "middle of the trilogy" syndrome. It's not as fresh as the earlier books, but it still doesn't conclude anything.
I think I'd like to read more books about Compton and Bayta. The problem is, however, that I'm getting tired of the Modhri.