The fourth series in the Dresden Books details a bit more about the nature of Fairies in the Dresdenverse. It also ups the ante in the perils that he faces, from simple cases that will ruin his life, to the potential long term health of the world. In short, it is a logical step from the last book, in which Harry took a step into a much larger world. We also learn a bit more about the politics of the world and the city at large.
I generally liked the book, though I admit I liked Grave Peril slightly more. I think it was because, as much as I liked this book, Grave Peril was a quantum leap forward, and actually involved loss and sacrifice for the main character. I like it when actions have conseuqnces. I consider it the hall mark of a good story. Its one of the reasons I like Joss Whedon and J.J. Abrams. Not only is it hard for me to predict what's going to happen, but when you look back at their work, its clear that they've got a plan in mind, which means that they have a set of rules, and they stick by them.