First rate!! Like the best historical fiction I have enjoyed in the past, this novel is entertaining, a carefully crafted mystery on many levels, with interesting and engaging characters, and I found I learned a lot about the time and the place, Victorian England, and the London of Victorian England. Thomas De Quincey is at first not easy to like, but as the story unfolds I grew to appreciate him more and more. While his daughter Emily was terrific throughout.
Interesting introduction to Lord Palmerston, and marvelous denouement, " . . . all the efforts to save the city were coordinated through my office and that I myself personally directed the unmasking of . . ." Was it ever else?
I highly recommend Morrell's afterword at the end of the book, and the excellent review here on Goodreads by Lew Pendergrast.