But Mr. Stein, a former editorial writer and columnist for The Wall Street Journal and the author of four other books, promises more detail than he delivers on this front. '' 'Ludes'' soon shifts into the world of Lenny's predrug days. We hear about investments, foreign businessmen, Mercedes Benzes and tax advice. Not until almost halfway through the book do we learn more about Quaaludes, the sedative that came into prominence about 10 years ago as a dangerously powerful ''recreational drug.'' It soon found its way into the homes of many abusing adults, and thousands of teen-agers wouldn't think of attending an Ozzy Osbourne concert without taking one. Quaaludes are a mysterious and damaging drug.

'' 'Ludes'' is best when Mr. Stein is observing the seamy Quaalude underground, keenly showing us specifics like the difference between real 'Ludes and bootlegs, or the decor of those dingy apartments and ''doctors' offices'' where Lenny must go to get the drug: ''It was a small apartment. A pigsty. Empty lemonade containers and Hustler magazines were scattered all along the peeling linoleum floor in the tiny kitchen. The only uncluttered spot in the room was the kitchen counter. There was a professional Rohm and Haas triple-beam scale for weighing cocaine and 10 rows of Quaaludes, 10 in each row, in neat straight lines.'' Throughout his book, Mr. Stein makes quite a case for Los Angeles as the evil seducer that brings Lenny to his doom. No wonder. Mr. Stein's L.A. is an L.A. of brand names, designer labels, paunchy men squinting from ill-fitting contact lenses and raven-haired women who come up to him at parties and say things like ''You think anyone in this town likes anyone else who doesn't make money? You think there's anything in this whole town except people looking for what somebody else can do for them?''

'' 'Ludes'' winds up being largely about the high-finance business world and the destruction of an otherwise sensitive personality obsessed with material gain. ''In New York,'' Lenny's long-suffering wife, Linda, reasons, ''it was clearer that life went on even if you were not rich. But in Los Angeles, where the only value at all was money, where no worth attached to any other activity except making money and displaying money, there would naturally be terror in not having a great deal of money.'' Lenny and Linda battle failure and briefly escape to Santa Cruz, but Lenny tragically returns to Los Angeles, as a sort of young Willy Loman on Quaaludes.

'' 'Ludes,'' unfortunately, is also about restaurants. In the process of telling Lenny Brown's story, Mr. Stein manages to mention almost every exclusive restaurant in Los Angeles. It often seems that all the important conversations in the book take place over meals at Ma Maison, the Moustache Cafe, La Scala, the Palm or Le Restaurant. I just wish I knew more about how Quaaludes make their way into the hands of so many teen-agers, rather than the names of so many maitres d'hotel.

Still, '' 'Ludes'' tells a melodramatic tale well. Lenny dies, of course, in an all-too-quick drug shoot-out that Mr. Stein hears about on the radio late one night. It is a touching moment when he holds Lenny's dog, Daisy, while the policemen and coroner discuss his disastrous last drug deal. Mr. Stein accompanies the body to the morgue: ''I did not want him to be alone in his final ride through the city that had mystified and awed and overwhelmed him. We traveled along the Santa Monica Freeway toward downtown and all along the freeway, we could see the lights of Los Angeles, a city which Lenny had loved, and which could not possibly have cared less.''