After leaving the Pinkertons in 1922, Sam Hammett took to calling himself by his mother’s maiden name, and, as Dashiell Hammett, became a frequent contributor to the pulpy fiction magazine Black Mask. With those stories, he began to popularize the “hardboiled” style of American detective fiction. Although his most famous creation remains Sam Spade—the amoral private detective central to the action in The Maltese Falcon—Hammett’s first P.I. hero was the short, fat gumshoe known simply as “the Continental Op.”

Drawing on his experience in the Pinkertons, Hammett imbued his Continental Op tales with a type of gritty realism and working-class cynicism that stood in contrast to the prevailing structure at the time, which relied on the archetype of the gifted amateur. And unlike the cerebral Sherlock Holmes or the foppish Hercule Poirot, the Continental Op reads like a believable person, if only a little too tough for his own good.

When Hammett started writing his Continental Op tales in the early 1920s, the private-detective industry was in a transitional period. Along with the more-muscular federal agencies such as the Bureau of Investigation and the Secret Service, municipal departments in New York, San Francisco, and other American cities had undergone sweeping changes two decades before. Now larger, better equipped, and better educated thanks to civil-service reforms, professional lawmen began rendering large private detective agencies obsolete.

At the same time, private detectives were becoming affordable to the increasingly prosperous American middle class of the Jazz Age. Marital complaints, will disputes, and the protection of banks formed the meat and potatoes of private-eye work during the dramatic golden years of their fictional counterparts.

But the private eyes of the 1920s had less in common with their more-militarized, strike-busting forefathers than they did with the original P.I.’s of the early 19th century (or with their successors today, for that matter). Ever since the establishment of what is typically considered the first private detective agency in Paris in 1833, investigators had served clients who felt that the police were either unwilling or unable to do the work they required. The first private-eye celebrity, the former criminal-turned-police investigator Eugène François Vidocq, had opened his Bureau des Renseignements, or "Office of Information," after being pushed out of his public job during an internal overhaul that sought to clear out ex-cons.

Vidocq’s Office of Information quickly set about representing the interests of businesspeople and private citizens using the most advanced methods of early criminology and even rudimentary forensics. Its success incurred the wrath and distrust of the Parisian police, who hounded Vidocq with numerous arrests until the financial stress of repeatedly having to clear his name forced him to consider selling the agency in the 1840s.