The Sign of the Four
The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring the fictional detective. As with the first story, A Study in Scarlet, produced two years previously, The Sign of the Four was not particularly successful to start with. It was the short stories, published from 1891 onwards in Strand Magazine, that made household names of Sherlock Holmes and his creator.
It presents the detective's drug habit 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.