Dashiel Hammet: The author of hard-boiled detective stories and novels started out as a private detective. His first case? To track down a thief who had stolen a Ferris Wheel.
"Sam," as he was known before he began writing, left school when he was 13 years old and held several jobs before working for the Pinkerton National Detective Agency. He served as an operative for the Pinkertons from 1915 to February 1922, with time off to serve in World War I. From Wikipedia
Among the enduring characters he created are Sam Spade (The Maltese Falcon), Nick and Nora Charles (The Thin Man), and the Continental Op (Red Harvest and The Dain Curse). From Wikipedia