With out a doubt my favorite book in the Dresden Files. It kicks off the meta-plot, it crams a ton of action, a good mystery, introduces some interesting new characters, and is just jam packed with awesome. This marks the turning point in the series. With out spoiling it, BIG things happen in this one.
Character-wise, we meet Michael and Thomas (and Michael's family) who all play a big long term role in the series. They're interesting in their own right. But what really blew me away about this was the roller coaster ride of watching Harry and his friends get put through the ringer in this one. The characters are the main strength of this series and author. Going through the events of this story with them is both satisfying and a real doozy.
Plotwise, you can sum it up as: And then Everything changed. The are a lot of events and plot threads in this one. So much so that some times it seems like it should occupy half the series or so. And it hits all the right notes. The action, the intrigue, and character all hit just the right notes.
For the plot elements themselves, I've re-read the series a couple of times know and there were still new things to notice. There's a lot of material there, even if you don't know it. This is Jim Butcher at his best.
The next book is Summer Knight