The Sign of the Four by Arthur Conan Doyle

The Sign of the Four has a complex plot involving service in India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts (the Four of the title) and two corrupt prison guards. It presents the detectives drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet. It also introduces Doctor Watsons future wife, Mary Morstan.