Exceptional. All of the Pitt series is good, but this may be one of the best. As an avid reader of mysteries, I found it wonderfully frustrating to get into the last chapter without actually knowing who the killer was. Better, Perry did it without cheating. Once you get to the solution, you realize that ever clue you needed was right there all along, and each false trail was masterfully done.
My only complaint in this book is that Perry padded large sections with irrelevant musings, and several were actually out of character for the person musing. There is a section with Thomas mulling all of the dead ends in a dispassionate mental voice that is completely different than in any other part of the series, and another with Charlotte using the same cadence, rehashing discarded leads. It feels as if her publisher came back and demanded about twenty more pages, and she scrambled to cram them into an otherwise tight and well-crafted book.
On the other hand, her mastery of the period is incomparable, tossing the assumptions and mundane details of the day into the story in a way that draws you fully into a remarkable and fascinating point in history. The characters are perfect and well-realized, as always, and this book introduces one of my favourite bit players--Aunt Vespasia, the Lady Cumming-Gould. Delightful, insightful, intriguing and unconventional, just like this book.