How many Sherlock Holmes novels did Sir Arthur Conan Doyle write?

Arthur Ignatius Conan Doyle (22 May 1859 – 7 July 1930) was a British writer and physcian. He wrote 4 novels about Sherlock Holmes and Dr. Watson.

"A Study in Scarlet" is told in first person by Dr. Watson, and describes his introduction in 1881 to Holmes through a mutual friend and the first mystery in which he followed Holmes' investigations. The mystery revolves around a corpse found at a derelict house in Brixton, London with the word "RACHE" scrawled in blood on the wall beside the body.

"The Sign of the Four" (1890), has a complex plot involving service in India, the Indian Mutiny of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards .

"The Hound of the Baskervilles" is the third of the four crime novels featuring the detective Sherlock Holmes. It is set largely on Dartmoor in Devon in England's West Country and tells the story of an attempted murder inspired by the legend of a fearsome, diabolical hound of supernatural origin.

"The Valley of Fear" is the fourth and last novel in the Sherlock Holmes series. It was first published as a book in February 1915. A story about a very mysterious murder in Birlstone, beautiful countryside in England. John Douglas of Birlstone Manor House is killed, and the mystery around his murder is mind-blowing and much more intense than any of the previous Sherlock Holmes stories.