BROKE AND UNEMPLOYED, with no idea of what he wanted to do in life, a young man sat on a bench in Lafayette Park, opposite the White House. He was discouraged and a bit desperate. Then suddenly "he heard his own voice. It said: 'You're going to be a writer!' "

The young man was James M. Cain. Since that memorable day in 1914, Cain never failed to heed his inner voice. He became a leading journalist of his time, wrote for H.L. Mencken and Walter Lippmann. In the 1930s and '40s his best novels -- The Postman Aways Rings Twice, Double Indemnity, and Mildred Pierce -- stunned readers with their "passionate sex and lurid violence." As films, they revolutionized Hollywood movie-making and transformed the taste of a nation. Even as an octogenarian, barely able to put pen to paper, Cain, still writing daily, published two novels. The sharpness may have dulled and the momentum slackened, but Cain never ceased to work. He was obsessed with writing; his greatest dread was that he would become an ex-writer.

Cain, by Roy Hoopes, a Washington journalist, is the first biography of James M. Cain. Certainly, a more detailed and comprehensive chronicle of a man's life can hardly be imagined. In addition to the sources available to any biographer -- letters, interviews with contemporaries, critical works, and the like -- Hoopes interviewed Cain extensively, and for three years even discussed his project with Cain, living as a near-recluse in Hyattsville, Maryland. Cooperating fully, Cain churned out such extensive memoirs, Hoopes comments, that writing the biography was at times more akin to editing an autobiography. The result is a tome of 700 pages, illustrated with 78 photographs.

Instead of this massive work, apologizes Hoopes, he would have preferred a slim, elegant volume, like the kind Cain carefully crafted, but "there was simply too much in Cain's long life to compress into a small book . . ." Indeed, Cain's 85 years were as fast-paced, full, and controversial as his novels.

Born in 1892, in Annapolis, Maryland, the eldest of five children, Cain graduated from Washington College at 17. He drifted from job to job until, having decided to write, he fell under the spell of Baltimore's literary panjandrum, H.L. Mencken, probably the most exciting journalist of his day, certainly the most voluble and entertaining talker. Mencken's American Mercury published Cain's early pieces, which, like Mencken's work, are sharply satiric and provocative. Mencken called Cain "the most competent writer the country ever produced." Later, through his mentor, Cain landed a job under Ralph Pulitzer and Walter Lippmann as human interest editorial writer for the New York World. For seven years, Cain wrote about topics as diverse as baseball and the Bible, music (about which Cain was an expert) and catfish sandwiches.

Although Cain looked and acted like the tough guys who people his novels -- his face was pockmarked, his hair unruly, his voice gruff, his drinking and eating habits excessive -- he had a very private side that he revealed only to a few. Cain worried about himself and his writing, felt he had failed. Three divorces caused him to doubt his ability to relate to women. He was childless. Often he was broke and scrambling to pay debts and alimony. His fourth marriage, to Florence Macbeth, a noted opera singer, lasted 19 years, until her death. This marriage finally provided Cain with marital fulfillment.

When the New York World was sold in 1931 and Cain lost his job, he left the East for the glitter of Hollywood, where writers commanded exorbitant salaries. There Cain became famous--but not as a scriptwriter. Though his name is usually identified with the box-office successes of films adapted from his novels, Cain himself was a mediocre scriptwriter. His dialogue flopped; his plots floundered. From 1931 to 1948, even though Cain was often paid over $1,000 a week, he received credit for only three film scripts.

Cain's novels made him a Hollywood celebrity. Because of them, he was a hot property in movie land, was hired and rehired by at least nine different studios. But Cain did not enjoy the hoopla of Hollywood. He preferred intimate evenings with friends to snobbish Beverly Hills parties. On many such intimate evenings Cain and his friend, scriptwriter Vincent Lawrence, discussed what Lawrence called the "love rack." Later Cain quipped he hadn't the faintest idea whether the love rack was a peg on which lovers hung "the shining cloak of romance" or a torture rack.

Cain adapted Lawrence's principle and based Postman, which he dedicated to Lawrence, on it. The book's impact was explosive. One critic labeled it "unlaydownable." "Postman," Hoopes writes, "was probably the first of the big commercial books in American publishing, the first novel to hit for what might be called the grand slam of the book trade: a hard-cover best seller, paperback best seller, syndication, play, and movie. It scored more than once in most of these mediums and still sells on and on, even today." Recently Hollywood released another film version of Postman, starring Jack Nicholson.

As best seller followed best seller, Cain became the country's most reprinted author. His hard-hitting stories traced similar plot lines: a tough guy falls in love with an even tougher woman; both conspire to get what they want and usually -- because of murder, incest, homosexuality -- both are destroyed by their dreams come true. Cain's frank portrayal of taboo themes shocked the reading public.

Hoopes' book chronicles every detail of Cain's life: his childhood on Maryland's Eastern Shore; war service; teaching at St. John's College; work in the coal mines of West Virginia; Cain's attraction to low-lifers, from whom he learned the richness of colloquial speech; Cain's travel and meticulous research; his many unwritten novels; his painstaking revisions; his devotion to Florence, whom he nursed for several years before her death; and finally, Cain's last years, lived in seclusion, until his own death in 1977.

If Cain has a flaw, it is the book's lack of critical evaluation. Just what is Cain's place in American literature? He has been called "the poet of the tabloids," "hard-boiled," "almost a first-class writer." How good is he? What has he contributed? His vision is ironic, violent, pessimistic, even tragic, if one adopts the definition offered by Cain's father: "Tragedy is that force of circumstances driving the protagonist to the commission of a dreadful act!" Cain's dialogue is brisk, lean and colloquial, very much like that of Hemingway and Hammett, neither of whom Cain claimed as influences. His subject is Man; Cain refuses to philosophize. Above all, he is a masterly storyteller.

Cain wrote most successfully in the first person; to create, he had to pretend to be someone else, had to assume a persona. Why could he not write in the third person? Cain provides several answers that Hoopes presents, but Hoopes refrains from judgment.

Many other questions arise. Did Cain live up to the early promise of Postman? Did his journalistic training stymie his creative impluse? In short, as David Madden asks in James M. Cain, his excellent study of Cain's novels, published in 1970: "How much recognition does James M. Cain deserve? -- what is a just estimation?" Twelve years later Hoopes, while assembling a superb biography, avoids Madden's question, answering that posterity will judge Cain's worth. Hoopes, however, is part of Cain's posterity.