The Sign of Four was the second novel featuring which famous fictional detective, following A Study In Scarlet?
The Sign of Four (2001 film):  The movie is based on Arthur Conan Doyle's second Sherlock Holmes story. Arthur Conan Doyle:  Originally a physician, in 1887 he published "A Study in Scarlet", the first of four novels about Holmes and Dr. Watson.
Sherlock Holmes