Arthur Conan Doyle 
The Sign of the Four [EPUB ebook] 

The story is set in 1888. 
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 detective’s drug habit and humanizes him in a way that had not been done in the preceding novel,  
A Study in Scarlet (1887). It also introduces Doctor Watson’s future wife,  Mary Morstan.