Penzler has been involved in the world of mystery fiction for more than 30 years—as publisher, editor, bookseller, agent. Currently, he has publishing imprints with Harcourt in the U.S. and Hutchinson in the U.K., is the owner of The Mysterious Bookshop in New York, writes a column for the New York Sun and edits the annual Best American Mystery Stories for Houghton Mifflin.

What constitutes A mystery book? The question may not be as compelling as what is love or what is God, but coming up with an answer is not a lot easier. But let us venture this: a mystery is any fictional work in which a crime or the threat of a crime is integral to the theme or plot. So what if the two main writers' groups in the field can't agree on the term—in the U.S., it's the Mystery Writers of America, in the U.K., the Crime Writers Association. Although the words "crime" and "mystery" seem to be synonymous here, they certainly have different definitions, and several variations.

The literature of crime goes back at least as far as the Bible, where Cain slew Abel. Today, mysteries come in many different shades. The category caters to different tastes, and is the haven for a host of storytelling approaches, many with their own traditions. Although the categories and subgenres below have their borders shrouded in fog, let's recall Arthur Conan Doyle's reminder that "there is nothing more deceptive than an obvious fact."

The Detective Story

Edgar Allan Poe started it. Although it is true that there were many stories of crime, mystery, murder and suspense written before 1841, the year of "Murders in the Rue Morgue," those authors did not intend for the crime and its solution to be the story's raison d'être. Poe's tale established the form for more than the next century-and-a-half. Quite often, the traditional detective story stars a gifted amateur—C. Auguste Dupin, in the Poe tale—with Lord Peter Wimsey, Ellery Queen, Miss Marple and Perry Mason following suit. There are usually red herrings, innocents wrongly accused, and the shocking revelation that the least likely person is the culprit.

The Private Eye Story

The private eye replaces the amateur detective to produce a form nearly identical to the traditional detective story. While it is common to credit Dashiell Hammett with creating the PI story, he was, in fact, preceded by Sherlock Holmes a half-century earlier. Hammett did, however, make popular the hard-boiled (or tough guy) private investigator, first with the Continental Op, then Sam Spade and Nick Charles. For a while, back in the 1930s and '40s, when the police needed help they would (reluctantly) seek it from a smart-mouthed PI. Nowadays, the PI is more likely to intrude, though warned off by the cops. Current private eyes include Robert Crais's Elvis Cole, Robert B. Parker's Spenser and Lawrence Block's Matt Scudder.

The Hard-boiled Detective Story

An American invention, this sub-genre has produced much of the great literature of the 20th century. Hammett, Raymond Chandler, Ross Macdonald, Mickey Spillane and Robert B. Parker all reside in the pantheon, with Lehane, Grafton, Block and Paretsky ably working the beat.The Police Procedural

This category of mystery, which features official police work, has a distinguished ancestry—Wilkie Collins's Sergeant Cuff, Erle Derr Biggers's Charlie Chan, Georges Simenon's Inspector Maigret—but in recent times, the true procedural has become associated with not lone cops as much as entire police departments striving (sometimes at cross-purposes) to solve crimes. Contemporary writers whose work centers around the station house include Ed McBain, Joseph Wambaugh, James Ellroy, Colin Dexter (the Inspector Morse series), George Pelecanos, John Sandford, Linda Fairstein and Michael Connelly.

The Crime Story

When is a crime story not a mystery? When the criminal is telling the tale. Stories are generally told from the criminal's point of view and may include capers—like Donald E. Westlake's hilarious Dortmunder series; the more wry, hard-boiled novels of Elmore Leonard (in which it's often difficult to tell the good guys from the bad guys); or the truly noir, as in W.R. Burnett's The Asphalt Jungle and the books of Jim Thompson. Crime stories rarely have happy endings. The kings of darkness are James M. Cain, most famously in Double Indemnityand The Postman Always Rings Twice; and Cornell Woolrich, of The Bride Wore Blackand Rear Window.

Psychological Suspense

These are novels of fear and dread, in which the chilling effects become more pronounced as the plot progresses. The cause may be imagined (as the protagonist is often assured) but is more likely the result of a campaign by an unknown enemy. There is seldom much detection, and some of the stories verge on the brink of horror and supernatural without actually plunging in. Think Psycho, Diabolique, Don't Look Now and Suspicion.

The Thriller

In England, just about any mystery that involves something more active than a gentle poisoning in a vicar's garden is called a thriller. But in the U.S. it means a tale of espionage, international intrigue, political, religious or medical adventure. Rarely is much detection involved, and the action generally takes place on a large stage. Thrillers are usually fast paced, with the hero or heroine navigating a perilous obstacle course to reach an objective that almost always begs the question: was it worth it? Tom Clancy, John Grisham, David Morrell, Clive Cussler, Lee Child and James Patterson are a few of the bestsellers who practice in this arena.

Hard-boiled and Cozy

Hard-boiled novels, which tend to be targeted chiefly to male readers, feature macho protagonists (even if females), violence, bad language and sex. Cozies are aimed more towards female readers, with murders generally committed off-camera. Obscenities rarely exceed the "drat" level, and if anyone ever has sex they certainly don't talk about it or share the moment withreaders. Among the best-known cozy writers are Agatha Christie, Dorothy L. Sayers and such contemporary practitioners as Laurie R. King and Alexander McCall Smith.

Categorizing all the evidence for different forms of mystery, however, doesn't solve the mystery of mysteries entirely. One can line up the usual suspects only to find it has left out the many distinguished writers of the past 100 years who are not known for writing mysteries but who wrote supreme ones: William Faulkner (Sanctuary, Intruder in the Dust), Joyce Carol Oates (everything?), Norman Mailer (Tough Guys Don't Dance), Ernest Hemingway (To Have and Have Not), Joseph Conrad (The Secret Agent) and on and on. A shorter list might be the books that are not a mystery—completely crime-free, like Marilynne Robinson's Gilead. But that's a mystery, in a sense, too.