Ever since I was an English major at the University of Michigan, it has been my position that writers of mystery fiction have produced some of the finest and most enduring literature of the 20th (and now the 21st) century. In more than 50 years of voracious reading, I have experienced nothing to change my mind on that score.

It is with hard-boiled fiction that this literature bloomed most eloquently and colorfully. Ernest Hemingway was the most influential writer of the 20th century, but there is evidence to suggest he was influenced in turn by Dashiell Hammett, the first of the great hard-boiled writers.

James M. Cain, the quintessential hard-boiled writer, claimed he didn’t know what the term meant. He wasn’t alone. So what is it?

Hard-boiled stories mainly involve private investigators as the hero (though Sherlock Holmes was a private eye and the stories aren’t hard-boiled, and Cain never wrote a detective novel). They are also realistic, in the sense that people who go out and get a private investigator’s license are hired to solve crimes, which is more than the village vicar or the head of the gardening club can say.

P.I.s need to be tough, since they are dealing with killers, so in the books about them, they act tough and talk that way, too. They are loners, much like the old gunslingers of the West. They have a code of honor and justice that may not be strictly legal, but it is moral. They may be threatened, or beaten, but they won’t give up a case or betray a client. They are individuals, often matched against a corrupt political or criminal organization, but they prevail because they are true to themselves and their code.

STRUCTURE The private-eye novel has strictures tighter than a sailor on his first night of shore leave. In a narrative generally told in the first-person form, someone – frequently a young woman – comes to the office of a shamus because she’s in trouble. The police can’t or won’t help, or the situation is so sensitive that an investigation needs to be kept secret.

The dick takes the case, which is invariably about something more than he was told. He interviews people and learns secrets, frequently about events in the distant past. He is usually betrayed by one or more people, often his client – which, as he is a cynic, doesn’t surprise him.

By the time he concludes his investigation, there have generally been several more murders along the way as people attempted to keep secrets hidden. He turns over the culprit to the police, and continues with his lonely life, awaiting the next meager payday.

HOW TO TELL YOU’RE HOLDING ONE A snap-brim hat, a trench coat, and a frosted-glass office door may well appear on a dust jacket, even though they are utterly anachronistic. If a handgun isn’t illustrated, be surprised. The flap copy may include such words as “dame,” “P.I.,” “dick,” “shamus,” “for hire,” and “a case for.” The cover is probably dark, with a lonely street or storefront, possibly in the rain.

THE BEGINNING The hard-boiled detective was created in the pages of Black Mask magazine in the early 1920s by Carroll John Daly, a largely forgotten hack. He was immediately followed by Hammett, who brought real talent to the genre, giving it literary credentials. Daly also created the first private-eye series, starring Race Williams, and remained more popular than Hammett for more than a decade.

THE GREATS Hammett’s unnamed P.I., the Continental Op, appeared in numerous short stories and the author’s first two novels, “Red Harvest” and “The Dain Curse,” before he created Sam Spade in “The Maltese Falcon.” This remains, in many ways, the ultimate private-eye novel, imprinted on the memory partially because of the splendid Humphrey Bogart movie.

Raymond Chandler followed Hammett with his immortal Philip Marlowe in “The Big Sleep” and eight subsequent novels. A pure writer whose use of simile and metaphor has never been equaled, Chandler is one of the giants of 20th-century literature. When Ross Macdonald decided to write P.I. novels about Lew Archer, he emulated Chandler’s style as closely as he could. By adding Freudian psychology to the mix, he gave his novels a depth rarely achieved before or since.

Mickey Spillane, whose vigilante hero Mike Hammer made him the most popular writer in America for a decade, was the toughest of them all, and perhaps the best plotter as well. He was reviled by critics for his black-and-white views of justice, but readers loved his clarity of vision, and he became a great favorite of Ayn Rand.

Other outstanding hard-boiled writers include Erle Stanley Gardner (whose Perry Mason stories sold more than a 100 million books), Howard Browne (also writing as John Evans), Harold Q. Masur, Thomas Dewey, Jonathan Latimer, William Campbell Gault, Frederick Nebel, Paul Cain (his “Fast One” remains a towering achievement), Raoul Whitfield, and, later, George V. Higgins.

TODAY’S BEST There is no better writer today, in any field, than James Crumley, whose “The Last Good Kiss” can be reread endlessly with pleasure. Robert B. Parker has justly been placed in the pantheon of greats for his Spenser novels. Dennis Lehane is as good as it gets. Other terrific hardboiled writers, whose books may or may not feature private eyes, include Michael Connelly, Elmore Leonard, George Pelecanos, James Ellroy, Loren D. Estleman, and Lee Child.

Most of this column has been devoted to the hard-boiled private investigator, but hard-boiled prose has been employed by outstanding writers of suspense (such as Cornell Woolrich, who wrote the best suspense stories since Edgar Allan Poe); by writers of crime fiction who tell stories from the criminal’s point of view (such as James M. Cain, Jim Thompson, W.R. Burnett, David Goodis, and Richard Stark, a.k.a. Donald E. Westlake); and by those who write police novels (including Ed McBain, Ed Dee, Stephen Solomita, and Robert Daley).

Mr. Penzler is the proprietor of the Mysterious Bookshop in Manhattan and the series editor of the annual “Best American Mystery Stories.” He can be reached at openzler@nysun.com.