GODDESS: THE SECRET LIVES OF MARILYN MONROE by Anthony Summers. (Macmillan: $18.95, illustrated). Like a restless spirit, Marilyn Monroe keeps returning. Twenty-three years after her death, she is impersonated by Theresa Russell in "Insignificance," imitated by Madonna in the "Material Girl" video, reincarnated as Miranda Richardson in "Dance With a Stranger," and now cruelly dissected by Anthony Summers in "Goddess." This is the best of Monroe books, it is the worst of Monroe books. A biographer with an investigative reporter's sensibility and infinite patience, Summers has tracked down more than 600 witnesses and delved into government files and other untapped sources. He has made Monroe's private life public, recounting alleged amours with the famous and the infamous, and the final maelstrom of alcohol and sleeping pills. And he has discovered and published a photograph of Monroe after the autopsy, a photograph that is the ultimate horror. Summers is less interested in Monroe the complicated human being than he is in the myth. On his evidence, Monroe must have been the most compulsive nymphomaniac since Messalina. "Goddess" reeks with the sort of gossip that some will find delicious and others nasty. Many of Summers' witnesses are sleazy characters--hookers and hustlers and private detectives. Many remain unnamed. Many make statements about Monroe and her lovers that are absurd to those who knew her. On the basis of his witnesses, Summers draws a disparaging picture of Joe DiMaggio and Frank Sinatra. But what Summers deals out to Sinatra is nothing compared to his treatment of the Kennedys. He believes that Monroe was carrying on concurrent affairs with John F. and Robert F. Kennedy. Summers "documents" that Robert Kennedy was with Marilyn the last Saturday of her life, that he was in an ambulance en route to a hospital with his dying "mistress" and that he returned her corpse to the Brentwood house so that the subsequent accounts of her death became an elaborate cover-up. To believe him, the Kennedys are as bad as the Borgias. I don't believe him. If only "Goddess" were published as fiction, I could enjoy it with a good conscience. Unfortunately, it is offered as biography.