This is Book 2 of the series "The Chronological Sherlock Holmes".

"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. 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 "A Study in Scarlet" (1887). It also introduces Doctor Watson's future wife, Mary Morstan.

Sir Arthur Ignatius Conan Doyle KStJ DL (22 May 1859 – 7 July 1930) was a British writer best known for his detective fiction featuring the character Sherlock Holmes. Originally a physician, in 1887 he published "A Study in Scarlet", the first of four novels about Holmes and Dr. Watson. In addition, Doyle wrote over fifty short stories featuring the famous detective. The Sherlock Holmes stories are generally considered milestones in the field of crime fiction.