ONCE "MARY LOU" automatically brought to mind a song; now it's synonymous with "Olympic champion." The short girl with more bounce to the ounce, the "little buddy," as Bela Karolyi, her coach, called her, with the Wheaties grin, came off the gymnastics mat in Los Angeles in the summer of 1984 as America's newest darling.

Why? Charisma, for one thing. With most of the United States, not to mention the rest of the electronically connected world, watching on television night after night, Mary Lou Retton communicated the kind of 1,000-watt energy and girl-next-door wholesomeness that no other gold medal winner could match. It was love at first, second and third sight.

More important, Mary Lou Retton was the first American woman ever to win a gold medal in gymnastics, a sport that previously had been the province of the East Europeans. Together with her teammates, male or female, she emblazoned a new set of athletic skills on the American consciousness. True, Olga Korbut and Nadia Comaneci had thrilled us in previous Olympics, but they were foreigners -- "Commies," for heaven's sake! Is it any wonder that Karolyi's gym in Houston has tripled in size, or that every 8-year-old from Tampa to Tempe is begging her parents for gymnastics lessons?

How Mary Lou Retton, the runt of a litter of hyperactive kids from a humble family in West Virginia, arrived at such a level of mega- celebrity is the subject of this book. What makes it more interesting than one might expect (how interesting can a teen-aged gym rat really be?) is the way John Powers, a Pulitzer Prize-winning Boston Globe reporter, tells the dual story of both Retton and Karolyi. Powers, one of the country's most admired sportswriters, give us three voices: Mary Lou's, Bela Karolyi's, and his own. First, he introduces us in his own words to the two principals, then lets them take over in alternating chapters.

THERE IS A curious symmetry in the lives of the girl and her mentor.

Karolyi came from a coal-mining town in Transylvania, where his coaching genius built the remarkable Romanian team that captured the 1976 Olympics. His prize pupil? Nadia Comaneci. Mary Lou, lying on the floor at age 8 watching television in the coal-mining town of Fairmont, was entranced by the spectacle of Nadia in Montreal and began to dream of an Olympic performance of her own. Little did she realize she would wind up being coached by the same man who had created her idol.

She started in Fairmont by learning acrobatics at a dance studio, progressed in public classes held at a nearby university, and then began studying in earnest with a local coach. But eventually, both Mary Lou and her parents realized that if she were ever to reach her potential she would need much more specialized, intensive work.

Meanwhile, political back-stabbing and general dissatisfaction with the Romanian system led Karolyi and his wife and partner, Martha, to defect to the United States in 1981. The book is at its best when Karolyi is relating his saga of going from Romanian national hero, to penniless immigrant speaking fractured English, to hard-working proprietor of a gymnastics studio in Houston. Powers captures Karolyi's warm yet tough personality and even his expressive use of his adopted language. When Mary Lou arrives in Houston 14 months before the '84 Olympics, he sees in this "little bug" raw talent and the right attitude. "I could see the determination in her eyes. She is not giving up easy. Mary Lou is not the kind of person who makes hystericals."

The book presents Karolyi as the gymnastic equivalent of Ion Tiriac, another Transylvanian with a big mustache who has guided tennis stars Guillermo Vilas and Boris Becker. And Powers wisely switches to his own journalistic voice to describe in thrilling detail the culmination of the Karolyi-Retton collaboration. With Karolyi caged behind a barrier on the Pauley Pavilion floor because he is not the official U.S. team coach, Mary Lou vaults her way past Karolyi's former proteg,e, the Romanian Kathy Szabo, nailing a pair of "10s" and a place in our collective heart.

It remains to be seen whether Mary Lou, who has apparently not yet completed high school (she suspended her studies to concentrate on the Olympics) sticks to her announced goal of competing in the 1988 Games in Seoul. But even if she turns out to be a one- shot winner rather than an enduring champion, this two-person autobiography assures us that the American dream is very much alive, for pint-sized athletic girls and immigrants alike.