Turn Coat by Jim Butcher

In this book the presence of a Black Council that's working to undermine the world as we know it and unleash evil becomes increasingly clear. Harry has to reevaluate his relationships and his views of members of the White Council. I enjoyed watching Harry's innovative thinking in terms of magic, diplomacy, and mystery. This series builds well: I don't feel like it's either stuck to a mundane plot or adding weird new elements as the stakes rise. In short it's an organic and tantalizing development.