Traditionally, four novels have been placed inside the canon; they are as follows:
Published in 1887, A Study in Scarlet marks the introduction of the Sherlock Holmes character. The novel sees Holmes clash with an American, Jefferson Hope, as he searches to solve the murder of two mysterious figures.
Published in 1890, The Sign of Four sees Holmes investigate the pact between four convicts and two corrupt prison guards. The novel focuses on The East Indian company, exploring the consequences of the foreign invading London.
Published in 1902, The Hound Of The Baskervilles-- arguably the most popular of the Holmes' stories-- sees Holmes collide with the maniacal Jack Stapleton in an inheritance-driven mystery.
Published in 1914, The Valley of Fear-- which is set some time before the events in "The Final Problem"-- sees Holmes' first encounter with his arch nemesis, professor Moriarty.