The Laws of Murder by Chas. Finch

 This is one from the middle of the excellent Charles Lenox (Victorian PI) series, which I have been reading totally out of order. In this one, Charles has started his own private investigations firm, which is struggling. Then his cop friend Jensen is found murdered and Charles is called in to help Scotland Yard solve the case--and he uncovers a real can of worms. 

I love the characters in this series--Charles, his true love Lady Jane, his brother Edmund, his former butler now friend Graham and all the others who pop up from time to time. It's a charming series with interesting cases. And I do love how most of them are solved with old-fashioned footwork--though Charles's physician friend has started doing some forensic work for him, including studying fingerprints.

If you're new to this series, I would recommend starting at the beginning--though it's hard to tell where that is. A Beautiful Blue Death was the first one published but some of the later books explored Charles's early days as an investigator. In any case, they're enjoyable reads even if you go through them out of order.