I have read most of the books in this series, so it's plain that I generally enjoy them. Thomas and Charlotte Pitt, along with the others who regularly appear in these stories, are well-developed and plausible characters. By this time, they have become much like old friends and this familiarity adds to their appeal. Perry also does a good job of engaging the interest of the reader by providing mysteries that are intriguing from the start. Also, though I'm not an expert on Victorian London, she seems to do a good job of re-creating that milieu.
On the other hand, Perry sometimes makes it plain who the murderer is in her stories by giving you one clearly dysfunctional character. Sometimes, too, her stories virtually turn into morality plays. She will take up some social evil of the period, make it part of her plot, and dwell on it. Finally, Perry has a tendency to end her stories very abruptly, leaving loose ends dangling and making you feeling like you've just stepped off a cliff.
This particular installment has all the usual strengths. Charlotte and Emily work to solve the case and save Thomas from a dire fate. The unsolved death of Robert York three years earlier gets Pitt started. The case is re-opened because York's widow is soon to marry a Foreign Office official. York was also with the Foreign Office at the time of his death and some secret papers disappeared at that time, so any possibility of scandal or espionage must be put to rest. During the course of the story, the reader gets a close look at the evils of nineteenth century English prisons, but not more so than fits the story. The mystery deepens as more deaths compicate matters. Perry keeps the reader guessing right up to the end in this one. While the end comes rather abruptly, there aren't too many loose ends in this one, so the reader isn't left hanging so much as in some of Perry's other stories.
As a mystery writer, Perry is a step below Conan Doyle or Agatha Christie. Her stories are enjoyable, especially if you've read enough of them to be familiar with the main characters, but the plots aren't usually as difficult to solve. This particular episode, however, is one of her best. A first-rate whodunit that will keep you guessing to the very end. Give it a try.