By this point in the series, Butcher is starting (more) to repeat antagonists. Mavra makes another appearance, and it's certainly not her last (actually, she plays a significant role in the next book, Dead Beat too.) But what makes this book stand out most is the sex.
I don't mean actual sex scenes (so far, the only one I remember that even kinda sorta actually was on screen was with Susan several books ago.) Rather, this one focuses on the previously only hinted at White Court of vampires. While they are vampires, they really don't conform to any vampiric folklore the same way that the Black Court vampires do. Nor are they a slightly new twist on previous vampiric folklore the way the Red Court vampires are. Rather, the White Court is heavily based on folklore about succubi and incubi. They're the ultimate sexual predators, consuming the "life force" of their victims, and sometimes even killing them. They are the ultimate masters of Machiavellian manipulation and underhanded political intrigue, which makes them kinda fun.
Of course, they're also explicitly sexual predators, so the whole thing starts off when Harry is hired to protect the production team of a porn movie shoot that seems to be the target of a wave of curses. Despite this rather lurid set-up, Butcher doesn't really dwell on the sexuality; in fact, it's not really an element of the novel at all, except as a plot point, not something that's featured "on screen", so to speak.
In addition, Butcher takes pains to differentiate between the lust that the White Court vampires can inspire in their victims and love, which is anathema to them; something that can literally kill them in the same way that garlic or holy water is anathema to the more traditional, Stokerian Black Court vampires.
Sadly, no hint yet that we'll see what's the deal with the Jade Court; which Harry himself mentions that he didn't even know anything about before Shiro mentioned it offhand in Death Masks. Then again, there's a lot more territory to cover before Butcher finishes this series, so maybe they'll come up yet.
Some major plot points are introduced, which have long-lasting effects on the series beyond this point, but I won't get into them here just in case you haven't read this far yet. Needless to say, they're interesting. In particular what I find intriguing about the Dresden novels is their curious blend of episodic and serial nature. Each book is self-contained, and could stand on its own. The story arc of each novel is closed off. However, a number of other things carry forward serial-like. They're never (or at least haven't been so far) the main plot, or "metaplot" of the series as a whole, but taken together, one does start to gradually and dimly emerge.
One of these days soon, I'll blog (probably on Scratch Factory) about the concept of serialized vs. episodic RPG play.