Jayne Mansfield was known as America's ''Smartest Dumb Blonde.'' But her seductive smile and eye-popping hourglass figure overshadowed the 'real' Jayne, who spoke five languages and was a concert violist and pianist. A woman ahead of her time, she was both a mother and a celebrated sex symbol. Driven to reach the heights of show business, her story reads like a fairy tale - but one with a gloomy side and an unhappy, tragic end. A British film critic once described her as we should remember her: ''She has been as good an actress as her bubble-and-squeak roles have permitted her to be. The lady is friendly, the lady is warm-hearted, the lady is always good for a laugh.'' Richard Koper is the author of ''Fifties Blondes: Sexbombs, Sirens, Bad Girls and Teen Queens.'' He works as a family therapist and holds an extensive collection of 1950's film memorabilia.