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.

A Study in Scarlet is a detective mystery novel written by Sir Arthur Conan Doyle, introducing his new characters, consulting detective Sherlock Holmes and his friend and chronicler Dr John Watson, who later became two of the most famous literary characters in detective fiction.
Conan Doyle wrote the story in 1886, and it was published the next year. The book's title derives from a speech given by Holmes to Doctor Watson on the nature of his work, in which he describes the story's murder investigation as his study in scarlet There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it.
The story, and its main characters, attracted little public interest when it first appeared. Only 11 complete copies of Beeton's Christmas Annual 1887 are known to exist now and they have considerable value. Although Conan Doyle wrote 56 short stories featuring Holmes, A Study in Scarlet is one of only four full-length novels in the original canon. The novel was followed by The Sign of the Four, published in 1890. A Study in Scarlet was the first work of fiction to incorporate the magnifying glass as an investigative tool.
Reception]
Reviewer Alex Baker noted that By later standards of detective fiction, 'A Study in Scarlet' violates a cardinal rule: there is no way for intelligent readers to work out the solution for the mystery by themselves. The very first time that the reader hears of Jefferson Hope is when Holmes arrests him as the murderer. Nor is any previous hint given of Drebber and Stangerson's Mormon background. However, at the time when it was written, this rule did not yet exist, and detective fiction in general was taking its very first steps (to which this book greatly contributed)