This is Book 8 of the Dresden Files.
I wish that I could go into detail about all of the reasons I really liked this book, but so much of this plot is truly a suprise. I don't want to ruin any of the fun.
The basic premise is that the first annual SplatterCon!!! is in town, a horror movie convention where horror movie villains are actually showing up to kill people. There is black magic afoot, and Harry is naturally right there in the middle of it, trying to solve the mystery.
As usual, what seems to be a simple case becomes infinitely more complex, as forces far beyond amateur black magicians are at play. Implications reach as far away as Faerie and as close to home as Harry's best friends. It really is a great book.
Harry and Murphy at least start to talk about what they are to each other, about whether or not there is or ever will be an "us." They come to a conclusion but I don't think anyone of them (Harry, Murphy, or even Thomas) actually believe it.
Each book has its own small errors in it. One book had Harry using an elevator during a city-wide power outage. This book said that a mini-van was "crushed as if with a wrecking ball" and yet two pages later everyone is piled into that same mini-van and driving down the street. Editorial errors aside, this book and this series is a real winner. I'm sad that I'm starting to run out of books.