WISEGUY; Life in a Mafia Family. By Nicholas Pileggi. Simon and Schuster. 256 pp. $17.95.

HENRY HILL lives "somewhere in America" now with his wife and teenaged children, running a business and living in a $150,000 house "in an area with such a low crime rate that garden-shed burglaries get headlines in the weekly press." The irony is considerable, though in telling Hill's story Nicholas Pileggi does not harp on it: Henry Hill lives in this crime-free community courtesy of the Justice Department's Federal Witness Protection Program, which set him up there in return for the testimony he has given, and continues to give, against his former associates in the Mafia. It has been powerful and useful testimony; as a former working mobster -- a "wiseguy," in Mafia parlance -- Hill "seemed to have access to all levels of the mob world," and his intimate knowledge of its workings has helped federal prosecutors win convictions of a number of ranking thugs.

Several of those convicted were Hill's good friends during his more than two decades as a Mafioso, but by Pileggi's account this has not fazed him in the least; his cool, blunt, authoritative betrayals are the price he has paid for staying alive, indeed are his only alternative to certain death at the hands of his former allies. In jail six years ago on drug charges, Hill had heard the bad news: "his own protector, Paul Vario, the 70-year-old mob chief in whose house Henry had been raised from childhood, was through with him; and James 'Jimmy the Gent' Burke, Henry's closest friend, his confidant and partner, the man he had been scheming and hustling with since he was 13 years old, was planning to murder him." The choice was simple: squeal or die.

Not merely did Hill choose to squeal, he wanted to tell his story in a book; that, after all, is what many unsavory characters eventually do. His attorney approached Pileggi, a reporter who has specialized in the underworld for years and has no patience with "the egomaniacal ravings of illiterate hoods masquerading as benevolent Godfathers." In Hill, though, he found something else again:

"Hill was a surprising man. He didn't look or act like most of the street hoods I had come across. He spoke coherently and fairly grammatically. He smiled occasionally. He knew a great deal about the world in which he had been raised, but he spoke about it with an odd detachment, and he had an outsider's eye for detail. Most of the mobsters who have been interviewed for books and articles over the years have been unable to detach themselves from their experiences long enough to put their lives in some perspective. They so blindly followed the mobster's path that they rarely saw any of the scenery along the way. Henry Hill was all eyes. He was fascinated by the world in which he had grown up, and there was very little about it that he did not remember."

The result of his recollections is a slender but amply detailed and entirely fascinating book that amounts to a piece of revisionist history. Though Hill left the mob with deep regrets -- "The hardest thing for me was leaving the life I was running away from. Even at the end, with all the threats I was getting and all the time I was facing behind the wall, I still loved the life" -- he declines to sentimentalize it. The picture he draws is a useful corrective to that painted by many others -- ex-mobsters, journalists, novelists, film-makers -- who depict the mob as a happy if fractious family in which bad things happen, to be sure, but in which such solid virtues as love and loyalty ultimately triumph, thereby emphasizing the essential humanity of the mobsters and their families.

THERE IS, to be sure, a measure of love and loyalty in the mob that Hill describes, though scarcely enough to prevent friends from murdering each other if cicumstances seem to require it, and there is "family" in the somewhat perverted sense that, as Hill's wife Karen puts it, "There were absolutely no outsiders." But readers looking for a romanticized Mafia in the style of Francis Ford Coppola will not find it in Wiseguy. The mob that Hill describes is exactly what any clear-eyed observer of its behavior would expect: a cynical, violent, avaricious, lawless environment in which human life and human attachments are, in the deepest sense of the word, meaningless. Though the accuracy of this picture can of course be disputed, it will not be here for two reasons: Hill's testimony has the clear ring of truth, and Pileggi's reputation is impeccable.

Hill was born almost 43 years ago in the Brownsville-East New York section of Brooklyn, the son of an Irish father and a Sicilian mother; it was "the kind of neighborhood that cheered successful mobsters the way West Point cheered victorious generals." Hill's family had no mob connections, but the boy yearned for them: "At the age of twelve my ambition was to be a gangster. To be a wiseguy. To me being a wiseguy was better than being president of the United States. It meant power among people who had no power. It meant perks in a working-class neighborhood that had no privileges. To be a wiseguy was to own the world. I dreamed about being a wiseguy the way other kids dreamed about being doctors or movie stars or firemen or ballplayers."

Hill's dreams came true, and then some. Paul Vario took the willing, eager youngster under his wing; before long the 12-year-old gofer had graduated to petty crime, then to a life in which "hustling and schemes took up every waking hour." It was everything he had wanted:

"There were always at least a dozen dirty deals afoot. Aside from his own indulgences, his expenses were almost nonexistent. He had no dependents. He paid no taxes. He didn't even have a legitimate Social Security number. He had no insurance premiums to pay. He never paid his bills. He had no bank accounts, no credit cards, no credit ratings, and no checkbooks other than the phony ones he had bought from Tony the Baker. He still kept most of his clothes at his parents' house, though he rarely slept there. . . . Like those of most wiseguys, the events of his days were so spontaneously assembled, so serendipitous, that he never knew where the end of the day would find him . . . . Henry was perfectly happy as a bachelor, taking whatever came up as it came up. His life was utterly unfettered."

Even after he married Karen Freid, a nice Jewish girl from Long Island, the unfettered life continued. For a long time his wife thought he worked as "a bricklayer and low-level union official," and was oblivious to the existence of Linda, his girl friend, "the ultimate luxury purchase" for a successful mobster. Eventually the real terms of her existence came clear to her, and once they did she tried to make the best of them, but she never permittedherself any illusions about mob generosity or loyalty. When Henry went to prison in 1974 on a 10-year sentence (he served less than four) for extortion, she quickly learned that she was strictly on her own: "I've read about how these guys take care of each other when they're in jail, but I've never seen it in life. If they don't have to help you, they won't."

After prison Hill got involved in ever-larger schemes and hustles; he also got involved with drugs, which eventually brought him down. When the cops finally got their hands on him, they realized they had "a bonanza . . . not a mob boss or even a noncommissioned officer in the mob, but he was an earner, the kind of sidewalk mechanic who knew something about everything." They made him an offer he really couldn't refuse, and now he's out there somewhere, set up in a life of respectability and ease by a grateful Uncle Sam. As Pileggi notes wryly at the end of his revealing and morbidly amusing account, "Thanks to the government for which he works, Henry Hill has turned out to be the ultimate wiseguy."