Vicious Grace is my favorite of the three books out so far in this series. You could also phrase this as: each book builds on, and is better than, the last.
Despite my not really liking the first book so much, Vicious Grace was what made me want to re-read it: remember that straightforward Kill The Bad Guy plot? They knew up front that killing the bad guy would dissolve all the bad guy's magical works; that was part of the point. (The other part was to avenge the dead uncle.) Vicious Grace involves one of these magical works: there's a hospital in Chicago doing double duty as a magical prison, and Bad Guy had a hand in some of the wards. Those wards are gone now - have been gone for a year, since Bad Guy's death - and as its inmate loosens its bonds, the hospital's becoming dangerous.
And it's Jayné's fault.
Vicious Grace involves facing hard truths: there are a lot of things that have happened in the three books that, when you put them together, add up to some seriously bad shit. But Jayné hasn't wanted to see the unpleasant truth; it contradicts everything she's always believed, always taken for granted. And because Jayné hasn't wanted to see it, and it's a first-person narrative, the reader only sees it indirectly: you see all the little details too, but you get to choose whether to put them together into realizing what the bad shit is, or whether to believe as Jayné has believed.
At the end of the book, Jayné's eyes are open; the very last line is one of the two major reveals of the story. That one is a truth I sort of saw coming, but Jayné's perspective on the issue wasn't something I'd considered until she opened her mouth and said it out loud - and I realized that if I were actually in her shoes, I would be huddled under a blanket, hyperventilating. I'm really, really looking forward to seeing what she does with the situation over the course of Killing Rites, and I am now a little impatient for it to show up on my doorstep next week.