I hardly need to convince you to read this after four previous Laundry novels and a handful of short stories. You're in or you're out. But I'm impressed with how the author has continued to deepen what was, at the beginning, a silly-concept zombies-and-Nazis procedural (with bureaucracy jokes). The jokes are still there -- committee meetings and security theater galore -- but the characters have accumulated real wounds. Both physical and psychical. They accumulate more; this is definitely the descent-into-Hell (or into CASE NIGHTMARE GREEN) part of the series.
Of the last book, I said that Stross had figured out how to bring real-world human evil to the forefront, with the supernatural elements as thematic support. It is now clear that this is a conscious turn for the series. We have vampires, but the notion of vampirism is grounded in several directions: financial shenanigans, addiction, predation (maybe you're a vegetarian; I'm not), and the brand of glamorous sociopathy that we call "being rich". Stross is careful to show us lots of vampires, and they are not all evil, all good, enemies of the Laundry, friends of the Laundry, or any other simple category. The vampires are, you know, human.
This being a Laundry novel, it is a spy-agency plot along with everything else. And there are schemes and plots and schemes on top of those. Perhaps too densely packed; the narrator has to go back and explain it all to us, from his post-facto viewpoint, a couple of times. But that's okay. It's not primarily a spy-agency plot. It's... I guess it's an episodic character drama set in a goofy-slash-horrific secret-history urban fantasy setting. Which is to say, it's the same model as several different television shows that I'm addicted to. No wonder.