The question of what exactly is a mystery may not be as compelling as what is love or what is God, but it is no less a mystery.

Although I have been reading mysteries for more than half a century (good grief! I think I need to stock up on Depends) and have been involved professionally in the world of mystery fiction for more than 30 years as a publisher, editor, bookseller, literary agent, reviewer, anthologist, historian, essayist, and all-around know-it-all, I can’t actually define it.

The professional writers’ organization in the United States is called Mystery Writers of America. In the United Kingdom, it is the Crime Writers’ Association. Though utterly different, the words “mystery” and “crime” are used interchangeably by the people who create them.

Being a big-hearted, all-inclusive sort of guy, I define mysteries broadly to include any fictional work in which a crime or the threat of a crime is integral to the theme or plot. There are many ways to tell a story, and the mystery genre is rich in the possibilities of presenting a crime.

Here are what I think of as the major sub-genres. It must be noted that the boundaries of all these are as clearly defined as fog, that many overlap, and there is not another soul on the planet who will agree with these definitions.

THE DETECTIVE STORY

This traditional narrative is how many readers define mystery fiction. It was essentially created by Edgar Allan Poe in 1841 with “Murders in the Rue Morgue,” which established the customs of the genre for all time. A murder is committed in an apparently sealed room, and the police are so baffled that they seek help from the brilliant amateur criminologist, C. Auguste Dupin. He has a sidekick who asks him the questions the reader would ask. Dupin makes careful observations followed by deductions that seem obvious once explained, then unmasks the killer, much to the amazement of one and all. (If this sounds like a Sherlock Holmes-Dr. Watson story, you’ve been paying attention.)

THE PRIVATE EYE STORY

In terms of story structure, an amateur crime solver may be replaced by a private investigator to produce a nearly identical traditional detective story. Al though many PIs are not tough guys (think of Holmes or Nero Wolfe), the majority have been. Dashiell Hammett’s Continental Op and Sam Spade, Raymond Chandler’s Philip Marlowe, Mickey Spillane’s Mike Hammer, Ross Macdonald’s Lew Archer, and Robert B. Parker’s Spenser are all hard-boiled detectives who reside in the pantheon of memorable characters in the history of literature. Such contemporary giants as James Crumley, Dennis Lehane, Sue Grafton, Lawrence Block, Sara Paretsky, Robert Crais, and James W. Hall give hope to those who enjoy these stories.

The private eye is one of the greatest creations of modern literature, as quintessentially American as the Western hero who has the strength and courage to battle for justice as a lone individual, regardless of the odds he must confront.

THE POLICE PROCEDURAL

A more realistic approach to the solution of crimes is a relatively recent invention – the police procedural. There had been many official police officers through the years (Charles Dickens’s Inspector Bucket, Earl Derr Biggers’s Charlie Chan, Georges Simenon’s Inspector Maigret), but they acted pretty much on their own, much in the manner of an amateur sleuth or private eye.

In a true procedural, as exemplified in the nonpareil 87th Precinct novels of Ed McBain, the entire police department is involved in a case: uniformed cops, forensic specialists, detectives, brass. Such television series as “Hill Street Blues,” “NYPD Blue,” “CSI,” and the first half of “Law and Order” all owe their existence to McBain and his creation.

THE CRIME STORY

Authors of crime fiction frequently deny that they are mystery writers – not without justification if a narrow definition is applied. These stories are generally told from the criminal’s point of view and include capers, gangster tales, and noir fiction. Do not expect a happy ending, except in the occasional comic caper of the type produced for the past 40 years by Donald E. Westlake.

PSYCHOLOGICAL SUSPENSE

These are novels of fear and dread, in which a central character experiences terror at several levels, the chilling effects becoming more pronounced as the novel progresses. The cause of the protagonist’s fright may be imagined or, more likely, the result of a campaign by an unknown enemy. The fear may be of madness or death, either to the main character or someone he – or, more commonly, she – loves. There is little detection, and some of these tales verge on the brink of horror and the supernatural.

THE THRILLER

In England, just about any mystery involving something more active than a poisoning in the vicar’s garden is called a thriller. In the United States, it means a tale of espionage, international intrigue, or political, religious, legal, or medical adventure. Rarely is much detection involved. These novels have greater force, a faster pace, as the hero or heroine tackles an obstacle course in which an objective is achieved at great heroic cost.

HARD-BOILED VS. COZY

Two terms that appear to be used almost exclusively in the mystery world are hard-boiled and cozy. Hard-boiled novels tend to be aimed more for male readers (if you’re a woman who likes them, please don’t write to tell me so). Protagonists are usually macho (including the males), and there is lots of described violence, frequent bad language (suggesting realism), and easy sex (suggesting fantasy).

Cozies tend to be aimed toward female readers (if you’re a man who likes them, I don’t want to hear from you, either). Murders are generally committed offstage, obscenities rarely exceed the “drat,” “darn,” and “fiddlesticks” level, and if anyone ever has sex they certainly don’t talk about it or share the moment with you.

So enjoy your next mystery, regardless of the subcategory. If it speaks to you, it’s the right book. And you don’t have to feel guilty about it – mystery fiction is not immoral, illegal, or fattening.

Mr. Penzler is the proprietor of the Mysterious Bookshop. He can be reached at openzler@nysun.com.