By Mercedes Lackey and James Mallory. Third book in the Obsidian Trilogy; the first two are The Outstretched Shadow and To Light a Candle.
I don't think this book is really worth reading unless you've already read the first two and want to know what happens. Actually, I read the first two and I still don't think this was really worth finishing. It is a little dense at times and not very compelling. The climax proceeds with rather terrible inevitability which results in a lack of suspense: you've been told what's going to happen since book one and (spoiler) it finally happens.
The ending is "happy" enough, but I don't think it's enough of a reward for slogging through this. The author cheats. Also, it seems like a lot that could go wrong doesn't, something I haven't noticed happening quite so obviously in other books. Others have a lot of threads and anguish tying up the third part of the trilogy, but this one seems to proceed rather smoothly, even if a lot of people die.
And now for a large, spoilerific whine about the ending: It's stated several times that the Wild Magic doesn't care about individuals. Therefore, one of the only reasons for it to, uh, "help" Idalia is because it plans to use her again in the future. The other reason I can think of is because others who hear her story in the future might not be so willing to serve if they saw how little she was rewarded. Since Idalia should no longer be able to use magic, the first reason doesn't really seem plausible to me, and even if it is true it means the Wild Magic is cheating more than death, but in addition the great covenants that it's suggested allow it to exist. The second reasoning is not so plausible since it seems like everyone in this book is terribly willing to sacrifice themselves without expecting personal reward at all. So, we are left with possibility #3: the author wanted a happy ending and didn't mind breaking rules to get it. Bleh.