THE MYSTERY, says the Oxford Companion to American Literature (in the four meager lines it allows for the genre), is a broad category of fiction ranging from Gothic romance to detective story. This last, it adds rather sniffily, is merely a "tale of ratiocination."

So it may be. But the fact is that crime literature is the most widely read class of fiction in just about any country you can name. And among the creators of that vast sea of villains and clue hounds, Ed McBain stands out as one of the great ratiocinators of our time.

McBain, whose legal name is Evan Hunter (alias Richard Marsten, alias Hunt Collins, etc.), was born Salvatore A. Lombino in New York City 70 years ago come October. His grandfather was a tailor by trade; his father, a postman; his mother, as fate would have it, a mailroom clerk at one of New York's major publishers, Harcourt Brace.

"I may have been very poor growing up," he says, "but you better believe I always had good clothes. I'd walk to my grandfather's shop after school every day, drink hot chocolate, sit by the sewing machines, and watch the men chomp on their cigars as they hunched over their work. I loved to hear them talk."

Perhaps it was there that he got his sharp ear for crisp, wry dialogue and his eye for a clear-cut pattern. But if he was taking in badinage and strategy, he didn't know it at the time.

What he did know was that like his father -- a swing band drummer in his off hours -- he had a strong artistic bent. In grade school he began to draw; by the time he was in high school he was the school cartoonist.

He attended Cooper Union for a year, studying art, but it was 1944, a war was on, and in order to avoid the killing fields of Italy, he joined the Navy and was shipped to the Pacific just as the United States began its occupation of Japan.

"I got the cushy jobs," he recalls. "Making signs, that sort of thing." But the experience, one he has yet to write about, made a deep impression on him.

"I was corresponding with a girl I knew from high school. She was at the University of Wisconsin studying English, and wanted to be a writer. One day it hit me that my letters were better than hers."

Bored with the limited subjects a ship could offer an artist, he started to write crime stories and, with the help of a former professor on board ship, honed his skills. By the time he left the Navy, he knew that writing was what he wanted to do.

He returned to study English at Hunter College and, when he graduated in 1950, got a job as a teacher at a vocational high school in the Bronx and began writing for the pulps. Teaching was a short-lived career, but years later it would serve as the cornerstone of his breakthrough book.

Married now and with a son and a new name, Evan Hunter sold lobsters and did what odd jobs he could until he landed a position at Scott Meredith, Inc., a literary agency whose market innovations would eventually change the face of American publishing.

But an office job didn't stop him from writing. He was so prolific, in fact, that he would send pulp editors three or four stories at a time, all under different pseudonyms. From time to time, magazines would feature a number of his creations in the same issue, and the editors would never know it.

At the agency, Hunter worked with many great writers of the day (even instructing the masterful P.G. Wodehouse on how to trim his books). But working with other people's writing was not what he really wanted to do. After two years, he left to write the book that would change his life, The Blackboard Jungle (1954).

SHORTLY after its publication, he was offered a three-book contract by Pocket Books, which was looking for a detective writer to replace Erle Stanley Gardner, the aging creator of Perry Mason. These three were the first of his famous 87th Precinct novels, which now number 47. Because the police procedural was seen as a cut below "real fiction" and because Evan Hunter was building a name as a "real writer," it was decided that the books would be published under the name Ed McBain. "You see, the mystery had fallen into disfavor," Hunter explains. "Put simply, it was considered trash. If I brought anything to this genre it was that you could dare to write a mystery well." Before long, the New York Times's Anthony Boucher was citing McBain's tart, taut little novels in his regular columns.

Ironically enough, Ed McBain's fame (a product of his 64 books, the most successful of which is Kiss) has always far exceeded that of Evan Hunter (26 books, the most successful of which is Mothers and Daughters). But even in his own mind, the two writers have very different personas. "As Ed McBain, I try bending and breaking rules all the time. I've even put a lead character into a coma in chapter one. That's hard to do! If I tried it as Evan Hunter, I'd fall flat on my face."

A highly disciplined writer -- he writes 40 pages a week -- Hunter has dabbled in movie script writing (Alfred Hitchcock's "The Birds," 1964), television scripts ("The Chisholms"), and science fiction (Nobody Knew They Were There, 1971). And, rather than giving himself up entirely to the resounding success of Ed McBain, he soldiers ahead with the more complex "straight novels" of Evan Hunter.

One thing he does singlemindedly, however, is plan his work. By the end of this month he will have published Hunter's Privileged Conversations, seen McBain's Ice made into a television movie, and plunged deep into his next 87th Precinct novel.

"So what will your tombstone say?" I ask, thinking this a shrewd way to get to the bottom of the man. "Evan Hunter or Ed McBain?"

"Evan Hunter, of course," he says, ratiocinating vigorously all the while. Even this he has planned to a T: "The name on top. And below it, a single line, very simple, very tight. He wrote like an angel'." CAPTION: Evan Hunter/Ed McBain