I am really torn about how to approach this review. On the one hand, the story is pretty good and the plot is solid. The characters all well-written and the pacing is spot-on. On the other hand, the author seems to have some moral blind-spots that are deeply troubling.
As with the first book, I really like the romance between Cady and Lon. They're in a more solid relationship, now, and Bennett does a good job handling a mature relationship without resorting to the typical slew of miscommunications and screw ups that less talented writers often indulge to “keep things fresh”. Cady's relationship with Jupe is a big part of the charm of Cady's relationship with Lon and it's good to see that grow as Jupe explores having, if not an outright mother figure, then at least a strong female authority figure who cares deeply about him. Jupe is deeply involved in the story, this time, and being there doesn't feel forced, strained, or manipulative at all. That's quite a feat and kudos to Bennett for pulling it off.
Unfortunately, where Bennett fails, she fails spectacularly. I've noticed a tendency with some authors to make moral choices be about outcomes rather than about choices. That's a natural tendency because moral choices are often put in terms of the expected outcomes of the choice made (pointing guns at people and pulling the trigger is bad because doing so will kill people.). While natural, it's also a peril-fraught, and ultimately lazy, way to view morality. After all, the decision to point a gun at someone and pull the trigger is immoral whether the person is killed or merely wounded or missed entirely. The choice to pull the trigger while the gun is aimed at somebody means you have made the choice to kill and that choice is evil regardless of outcome.*
The problem with authors who fall into the trap of thinking in terms of outcome rather than choice is that the author knows what the outcome will be (or, at the very least, what it won't be) and that can make them feel like a given act doesn't actually make their character evil. Cady, and this happens in the first third of the book, so I'm not going to bother flagging this as a spoiler, makes the decision to give a truly despicable man, Hajo, a half-ounce of her vassal potion. A single drop of this potion enables the administrator of it to command somebody, absolutely, for up to an hour. Frankly, the fact that Cady has a half-ounce already brewed and ready to go is a bit evil in the first place. A half-ounce is a heckuva lot of drops. These commands are completely unlimited and can include self-destruction. Hajo, bear in mind, is a drug addicted womanizer who, we are led to believe, only failed to rape Cady because she prevented him from doing so. Giving him the potion is a profoundly evil act and that evil attaches at the point the potion is given regardless of what Hajo eventually does with it. She accepts moral responsibility for all the things a person like him can do with it, not the things he eventually actually does. That Cady didn't agonize over this more seems like the author intruding because the author knows things Cady can't—namely what Hajo eventually will and won't do with the potion.
Cady giving Hajo the potion compromised my ability to identify with or have sympathy for her, at all. I'm sorry, but the situation just wasn't so dire that this choice was in any way justified. And she certainly didn't agonize over it as much as such a deeply evil action deserved. I have even less patience because I can think of at least three things she could easily have done to avoid it altogether (starting with giving Hajo a fake potion and having his test subject, Cady's devoted friend, pretend to be affected, and ending with having Cady and Lon kill Hajo once he'd done what they asked him to do—believe me, he needed killing, but even if they didn't want to go that far, she and Lon together were strong enough to simply take the potion away before they parted ways). Frankly, going the story route that required Hajo's knack of finding dead bodies in their investigation was a stretch to start with. That whole plotline was sketchy and having Cady commit such evil in its course was the capstone of a craptastic, unnecessary whole.
Add this to the whole Dare sub-plot (involving the hellfire club) and I get the impression that Bennett wants to put Cady in compromising moral situations where she'll have to make tough choices between right and wrong. Unfortunately, given how poorly Cady failed navigating those waters in this book, I have no confidence in Bennett's handling of moral dilemmas in future stories and find it unlikely that I'll be willing to continue reading them.
I'm sad to find such an overwhelming flaw in a series I was otherwise coming to appreciate quite a bit.
* Okay, so I don't actually believe that killing someone is always wrong. Some people need killing. If you don't think so, then UF as a genre isn't going to be your thing anyway and you might as well move along. We can argue about it if you like, however.