The hero, a eunuch detective named Yashim Togalu, lives in a moldering apartment and is comfortable in the sordid parts of the city, but he also has amazing access to all sorts of places, including the opulent royal palace and the restricted harem (after all, he is a eunuch). This allows the book to give a flavour of both the top and the bottom of the social scale in nineteenth century Istanbul. Since the author, Jason Goodwin, is an English historian and previously published a history of the Ottoman Empire, we can be pretty confident the details are all right.
The title refers to the Jannisaries, elite troops of the Ottoman Empire, who got too powerful and ended up terrorizing people. The Sultan had banned them ten years before the period of this book and there are concerns that they might be planning a resurgence. That's not necessarily how the mystery unfolds.
But it was a treat to read this book. And I'll be looking forward to The Snake Stone, the next book in the series.