The trend of the series over the last few books has been to circle the Black Council, rather than each mystery being, at least ostensibly, isolated. It's interesting to see how far the series has come from the original few books - how long has it been since Dresden has been to his office? At least the office gets a mention in this book, though for all Dresden's talk of money woes, it seems interesting that he keeps paying rent for it when his time seems so devoted to Warden matters lately. 
 The mystery itself was a bit of a let down. When Dresden notices a detail he wouldn't ordinarily notice, and then mentions it at least twice in different parts of the book, it becomes far too clear who the traitor was going to be. I don't try to guess the endings to mysteries, and I like to let myself be blown away in the reveal. So for me to know who the traitor is as soon as he comes on stage is really quite telling. 
 Still, the story is good. Dresden makes heavy sacrifices, and the characters are changed by the events of the book. In a series, that is generally a very good thing, and something that Butcher is handling better than most authors. 
 Perhaps the most interesting aspect of this books is all the extra insight we get into the White Council. We're no longer bound to Dresden's rather slanted view of them, and instead get some frank explanations from people who are more knowledgeable about and invested in the Council. It adds a great deal of nuance, and moves us away from viewing the Council as strictly an antagonistic force. 
 This was an excellent addition to the series, and it's a pleasure to see Butcher grow as an author.