Lord John and the Private Matter by Diana Gabaldon

This book features Lord John Grey, a minor character in Gabaldon's Outlander series, faced with two dilemmas that initially seem unrelated: he discovers the man who is engaged to his cousin has a venereal disease and he's asked to investigate the murder of a soldier who may be a spy for the French. As Lord John investigates and ponders what to do, it becomes clear that the mysteries are linked. This book is better at the historical fiction side of things than the mystery side of things.