The Valley of Fear is the fourth and final Sherlock Holmes novel by Sir Arthur Conan Doyle.
It is loosely based on the Molly Maguires and Pinkerton agent James McParland. The first book edition was published by George H. Doran Company in 1915. The story starts with Sherlock Holmes receiving a ciphered message from an agent of Professor Moriarty but without the key to the cipher. The book is divided into two parts: The Tragedy of Birlstone and The Scowrers.