A Study in Scarlet is an 1887 detective novel by British writer Arthur Conan Doyle. The story marks the first appearance of Sherlock Holmes and Dr. Watson, who would become the most famous detective duo in English literature. The book's title derives from a speech given by Holmes, a consulting detective, to his friend and chronicler 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 Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes by British writer Sir Arthur Conan Doyle. Doyle wrote four novels and 56 short stories featuring the fictional detective.