Turn Coat is book eleven in the Dresden Files series. The black council has been spreading it's evil influence for the last few novels in this series, but it stayed in the shadows as much as possible. This time they kill one of the leaders of the white council.
Harry finds himself in the thick of it when the main suspect comes to him for help. It's his old nemesis, the warden Morgan. Harry has to clear Morgan's name, keep his from being put on the list, and try to expose the real culprit while staying alive. There are some real surprises in this book. Not all pleasant. Very highly recommended.