A sentimental, melodramatic, but brief and likably unpretentious biography of entertainer Bobby Darin--who, though known during his lifetime for his brashness and ego, emerges now in a near-saintly light. (DiOrio had the cooperation of Darin's mother, sisters, and brother). Born Walden Robert Cassotto in the Bronx, illegitimate, Darin was raised by his grandmother--not learning till he was 30, says DiOrio, that his real mother was his ""sister"" Nina. But though this soap-operatic moment may be a bit hard to swallow, the other dramatic grabber here was obviously quite real: childhood bouts with rheumatic fever left Darin with a predicted life-span of 21-35 at the very most. Understandable, then, that he was driven to early, restless ambition: teenage flings at acting, then songwriting, then (by chance) singing--with lots of seedy touring before his first rock-novelty hit, ""Splish Splash,"" at 22 (thanks partly to Dick Clark and TV exposure). But, determined to be a ""legend"" by 25, Darin worked to go beyond the rock-'n'-roll scene--with his up-tempo ""Mack the Knife,"" recordings of standards; nightclub work with demanding mentor George Burns, and movies (most notably, Stanley Kramer's Pressure Point). His time on top was brief, however. And in the late Sixties his marriage to Sandra Dee deteriorated (she hated nightclubs), he went through an unwise ""denim and peace songs"" period, and later comebacks were sabotaged by worsening health--heart surgery, steroid medication, long hospitalizations. . . and, finally, grim deathbed scenes. DiOrio makes no attempt to assess Darin's achievements or consider his anomalous place in pop-music history (except to declare him ""a musical genius"")--but nostalgic fans will find this easy, good-hearted reading, and students of pop-culture (circa 1958-1965) may find a few intriguing tidbits.