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The Valley of Fear is the fourth and final Sherlock Holmes novel by Sir Arthur Conan Doyle. It is based on the supposedly real-life exploits of the Molly Maguires and Pinkerton agent James McParland. The story was first published in the Strand Magazine between September 1914 and May 1915, and the first book edition was published in New York on 27 February 1915.

The Valley of Fear, notable for Professor Moriarty's involvement, is set before "The Final Problem", the short story in which Moriarty was introduced. This introduces a logical difficulty, as in The Final Problem Dr. Watson has never heard of Moriarty, whereas by the end of The Valley Of Fear he is, or should be, familiar with his name and character.

Sir Arthur Conan Doyle (1859-1930) was a Scottish physician and writer, most noted for his stories about the detective Sherlock Holmes, generally considered a milestone in the field of crime fiction, and for the adventures of Professor Challenger. He was a prolific writer whose other works include science fiction stories, plays, romances, poetry, non-fiction and historical novels.

Sherlock Holmes is a London-based "consulting detective" whose abilities border on the fantastic, Holmes is famous for his astute logical reasoning, his ability to adopt almost any disguise, and his use of forensic science skills to solve difficult cases. Holmes, who first appeared in publication in 1887, was featured in four novels and 56 short stories.
Novels: A Study in Scarlet (1887), The Sign of the Four (1890), The Hound of the Baskervilles (1902) and The Valley of Fear (1915).

Short stories: The Adventures of Sherlock Holmes (1892), The Memoirs of Sherlock Holmes (1894), The Return of Sherlock Holmes (1905), His Last Bow (1917) and The Case-Book of Sherlock Holmes (1927).