The Inheritance by Chas. Finch

 Victorian detective Charles Lenox gets a note from an old schoolmate named Leigh implying that he's in trouble and needs Lenox's help. Then he doesn't show up for their planned meeting and can't be found. When Lenox eventually tracks his friend down, he finds out that someone is trying to kill Leigh. With the help of a cop named Frost, he sets out to find out who wants to kill his friend and why. There is also another case that John and Polly (from Charles's detective agency) are working on involving a broken window in the building where Parliament meets--that turns out to be more serious than was first thought.

If you like the rest of this series, you will enjoy this installment. Good story, good characters.