By Maria Snyder. Third book in the series, after Poison Study and Magic Study.
Warning: Grumpy review ahead.
This book was disappointing. I don't know if it's because I've had a couple months to cool off about the series or if I've just become more critical again, but somehow it lacked the emotional resonance the first two books had to support the plot. The plot was weak! OK, I was surprised about who the villain was, but I don't buy a certain character not dying under the circumstances. Instead of making the plot more believable (by having people's actions be more believable), it seems like the author added new types of magic and other gadgets, possibly to distract the reader with shiny.* And a long-standing mystery is cleared up in an offhand way in the last three pages, probably because the next book is scheduled to take place five years later (in fiction) and focus on a different character. Why not just leave it a mystery? People in real life don't get all the answers. Sometimes relationships end, friends part, and they never get a good reason from each other.
I think it could probably have used more editing to tighten up the story. It just seems loose, and while I finished it, it hasn't left me real enthusiastic about forthcoming books in this series.
Also, the sex was too much in my face, even though it wasn't explicit. And, despite that they're said to be in love, Yelena's relationship with Valek seems more about her than him. (SPOILER: She has a stupid moment where she falls apart.)
* Part of the fun of science fiction and some fantasy is doing a lot with a little. Adding thousands of kinds of magic to a world without a good reason is not so much, and worse, it is done in a distracting rather than an interesting way. (Of course, to be interesting, it might require a good reason.) On the other hand, books like Daniel Abraham's Shadow in Summer do impressively much with one (central) magical conceit (poems can harness elemental forces, like sterility, to human wills—but there is a price).