WHAT EVER HAPPENED TO BABY PEGGY? The Autobiography of Hollywood's Pioneer Child Star, by Diana Serra Cary, St. Martin's, 347 pages, $25.95.

"Honey, do you realize you're the youngest self-made millionaire in the entire history of the world till now?" This was a movie magazine reporter's question to 3-year-old Baby Peggy who, back in the days of silent films - long before Macaulay Culkin, even before Shirley Temple, Mickey Rooney and Judy Garland - set the style for the modern shameless exploitation of the child star.

At the time, of course, Diana Serra Cary (her name today) had no idea that she and her stagestruck parents were forging the grim, gothic context for all the tiny breadwinners - everyone from Gary Coleman to Michael Jackson and Drew Barrymore - who'd follow. (The first celebrated child-star actually was 4-year-old Cordelia Howard, who played the original Little Eva in the stage version of "Uncle Tom's Cabin" in 1853.)

Baby Peggy grew up in and with the Hollywood movie industry. She was famous as a child and is virtually unknown today. Her biggest film, "Captain January," was later remade with Shirley Temple.

This book begins novelistically. Cary describes Baby Peggy's successful audition clinically, from an omniscient point of view. She shifts to the first person with the first line of Chapter 4: "My workday - six days a week - began shortly after seven-thirty every morning with the punctual arrival of my father and myself at Gower and Sunset (Boulevard)."

The author has an eye for detail. She starts with the derivation of "Hollywood," which was named after a Spanish toyon bush that flourished in the area. The details enrich this story, making it more literate than the traditional star autobiography.

"Baby Peggy" also has the brisk pace of a two-reeler, enhanced by the author's discerning eye, memory and reporting skill, and wry tone. She also is funny. Her mother is "plain-looking . . . her figure had fewer curves than the road to Bakersfield." Julius Stern, an immigrant movie producer who preceded and anticipated Sam Goldwyn's malaprops, wanted to call his first studio Miracle Pictures so that he could use the motto: "If it's a good picture, it's a Miracle!" To critics who disparaged his two-reel comedies, Stern angrily responded: " comedies are not to be laughed at!"

As she tells her story, she also shows how the industry disassociated itself from its exploitation, which is how and why studio chieftain Hal Roach could look right through George (Spanky) McFarland, who was presenting him with a Lifetime Achievement Award. McFarland was desperately hoping for a word of acknowledgment from the man who'd owned him body and soul during the heyday of "Our Gang" and "Little Rascals" comedies.

Parents disassociated themselves, too, from what they were doing to their children. Darla Hood, another child staple of "Our Gang" and "Little Rascals" comedies, gave her parents a life of ease and a handsome home on her $600-a-week salary, of which she never saw a penny. Dying in only her 40s, she was in her casket when her mother sobbed over her: "Oh, Darla! Darla! You were such a good little girl and you had such a hard life!"

Cary notes that Shirley Temple's 1988 autobiography, "Child Star," divulged that her father had failed to make legally required deposits to her trust fund for eight years, handing out nonrepayable personal loans to relatives and friends.

"Baby Peggy" explains how Cary survived the wildly improbable early years as her family's meal ticket. With a $10,000-a-week contract at age 3, she was signed at 5 to a three-picture deal for $3.5 million, putting her on a plateau with Charlie Chaplin. By the time she was 7, her family had squandered the fortune she'd earned. At age 13 and destitute, she was striving for her third comeback, in talking pictures.

Her story was as maudlin and Dickensian as the plots of some of her pictures. Amazingly, like some of those pictures, it had a happy ending. Baby Peggy made an unwilling comeback, becoming a vaudeville headliner at $2,000 to $3,000 a week. Ultimately, she gravitated to a literary life, becoming in her adult life a writer (her previous books include "The Hollywood Posse," a history of Hollywood cowboys, and "Hollywood's Children").

She also became a trade book buyer for a bookstore. Her acting training enabled her to commit to memory a large store's entire inventory. And commit to memory as well, fortunately, the details of a sad and fascinating young life.