The Sign of the Four (World Classics, Unabridged) - Arthur Conan Doyle

The story is set in 1888. The SIGN OF THE FOUR has a complex plot involving service in East India Company, 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 detective's drug habit and humanizes him in a way that had not been done in the preceding novel.