The Valley of Fear by Arthur Conan Doyle

The plot of the novel is based very loosely on the real-life activities of the Molly Maguires and, particularly, of Pinkerton agent James McParland.
The novel is divided into two parts: in the first, Holmes investigates an apparent murder and discovers that the body belongs to another man; and in the second, the story of the man originally thought to have been the victim is told.