Conan Doyle Arthur. The Sign of the Four

The Sign of Four is the second detective mystery novel by Sir Arthur Conan Doyle featuring the fictional character Sherlock Holmes. The story was first published in serial form in Lippincott's Monthly Magazine in 1890.
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 (cocaine) and humanizes him in a way that had not been done in A Study in Scarlet. It also introduces Doctor Watson's future wife, Mary Morstan.