ELVIS AND GLADYS by Elaine Dundy (Macmillan: $18.95). Elvis Presley was an American tragedy, rising from Mississippi poverty to nearly unprecedented pop cultural stardom--but you know the story, right? The fact is, we don't know the real story at all. Most writers have concentrated (a la Albert Goldman) on sensationalism or social and musical significance. Elaine Dundy, author of two novels and a biography of Peter Finch, knows little about rock, but she was intrigued by the mysteries of Elvis' life. So she set out to do some actual research, reportedly interviewing more than 100 people who knew Elvis or his parents during their early days in Tupelo and Memphis. She comes up with fascinating, if sometimes belabored, details about family history and suggests that Elvis pursued a musical career with far more determination than has been assumed. So far, so good. But Dundy loses energy (or her sources) midway, turning from the original reporting to drawing sometimes absurd inferences and largely recycling what is already known, much of it highly critical of Col. Tom Parker's motives in guiding Presley's career. The relationship between Elvis and his mother, Gladys, is only randomly explored, leaving the suspicion that the title was an afterthought that offered the most commericial lure. In a strange way, the book ends up mirroring Elvis' own life. Despite early promise and detail, "Elvis and Gladys" is a somewhat careless and confused compromise.