IN NOVEMBER, 1963, a young, unknown Russian woman became a widow and a footnote in history when her husband assassinated President John F. Kennedy and two day later was himself shot by Dllas night club owner Jack Ruby.

Priscilla Johnson McMillan's very long book is based on extensive interviews with Lee Harvey Oswald's widow, Marina. The book offers few new details about the assassination or events leading up to it. Marina reiterates what she told the Warren Commission - that, in her view, Oswald acted alone, that he was not capable of being a third-rate spy, much less an international conspirator. McMillan, for her part, relies heavily on testimony and other published accounts for assassination detail, and on family and acquaintances for insight into Oswald.

The book, however, richly details a neurotic union between Marina and Lee, and fleshes out Oswald's personality as it recounts seemingly every conversation the couple ever had - from the revealing to the meaningless.

Oswald according to Marina, was a secretive, chronic liar, with delusions of grandeur and, above all, an obsession with fame. Subject to violent mood swings, Oswald could turn from cooing baby talk and tenderness of savagely beating his wife. As his personality disintegrated in the months building up to the assassination, Oswald beat Marina harder and with "the gleam of pleasure in his eyes.

Marina was a ready accomplice, goading Oswald into violence, taunting him for being an ineffectual lover.Russian fiends in Dallas urged Marina to divorce Oswald, sheltered her when she periodically left home, and gave her English lessons.

But Marina was not only trapped by her foreignness and fear of going it alone in America; she remained a strangely satisfied prisoner. She alienated the Russian community by going back on her promises and returning to Lee.The relationship, McMillan writes, "was founded on mutual willingness indeed a mutual need, to inflict and accept pain."

Marina's life holds all the stock agonies of melodrama - illegitmacy, a dying mother, a stepfather who beat her and denounced her as a whore. Her marriage was only one of many self-destructive routes she took. A bright teenager, she neglected her studies in pharmacy, ran around with the more rebellious youths, then rejected her Russian suitors for Oswald. Catching an American - and one who had that rarity, an apartment of his own - gave Marina much-needed esteem, and made her the envy of her friends. Two months after they met, they were married.

In 1959, Oswald had come to Russia on a tourist visa, and immediately announced that he wished to defect. He was eventually allowed to stay in the country - not as a citizen, but as a "stateless person" - and was sent to Minsk to work in a radio plant. He was accorded many privileges - the apartment, a topy salary - but McMillan dismisses conjecture that Oswald was a Russian-paid spy, and writes categorically that all foreigners received such favors.

One aspect of the Oswald affair never fully explained is why Marina, then 19, was allowed to marry an American and late permitted to come to the United States with him. The book sheds little light on why Russian officials so easily permitted the marriage.

However, the primary reason for allowing Marina to leave, McMillan contends, was that the Russians wanted to get rid of Oswald. The KGB viewed him as an embarrassment (Oswald had slashed his wrist when first told that he could not remain in Russia), and he was considered an "unsatisfactory and uncooperative worker of below-average skill," who was emotionally unstable.

Therefore, if allowing Marina to go "was the price of getting rid of Oswald, why not?" she writes. The State Department allowed Oswald's return in 1962 for much the same reason that the Russians wanted him out, according to the author. "It was potentially less embarrassing for the United States to have its unpredictables and malcontents at home . . ."

Once in America, Oswald's actions became more deranged and hostile. At one point, in the spring of 1963, he raced into their Dallas apartment and said that he had just shot right-wing General Edwin A. Walker, then frantically searched the newspapers only to find his bid for immortality thwarted. Oswald had missed.

A few days later she locked Oswald in a bathroom to keep from going out, pistol in hand, to have a look (as he put it) at Nixon. Still she did not go to the police. Today, Marina says she is haunted by her guilt that she might have saved Kennedy. "Had she informed on Lee then, he would have had a terrible fright," writes McMillan, paraphrasing Marina's thoughts. "And had he been convicted of the attempt, he might have been in prison in November and Kennedy would have beens aved."

McMillan exercise an author's prerogative to make assumptions, no matter how strained, about what might have triggered Oswald to shoot Kennedy. He felt that fate had chosen him when kennedy's route lead past the book depository where Oswald worked. He "may have felt he had something in common" with the sheriff in a favorite movie, High Noon - torn between love (of his family) and "duty" (to kill Kennedy). Oswald watched two violent movies on television one about a plot to kill a presidnet. An FBI investigator's visit to his wife set off Oswald paranoia. He placed mystical significance in the number three - and three times Marina had refused his request to return with him to Dallas the night before the assassination. He could have been jealous that Marina had an old boyfriend she felt resembled Kennedy. Even the Presdient's speaking out on civil rights and Medgar Evers's murder a few hours later "would have strengthened some half-conscious association in Oswald's mind between the President and death."

No one will ever known if any of these is accurate, because Oswald died revealing nothing and still claiming his innocence. Conspiracy buffs - who daily expound new theories - dismiss McMillan's book as a pro-Marina fantasy. On the other hand, those who believe that Oswald acted alone proclaim McMillan's book as definitive proof. It is not - nor, in fairness, could it be. There are too many unknowns, far more questions than this book can answer.

The author uses few direct quotations revealing Marina's insights or feelings. Instead, McMillan acts as a sort of ventriloquist for Marina; and the story is hardly aided by the sometimes ridiculous phrasing. But the central weakness of the book is the author's hammered-in-steel certainty in assessing motivation. McMillan - who has a master's degree in Russian studies and is a Soviet specialist - plays curbstone Freud with questionable qualifications and at times ludicrous results.

Marina herself still doesn't know why Oswald picked Kennedy, aside from his desire for frame. "He was a lonely person," she says, "He trusted no one. He was too sick. It was the fantasy of a sick person, to get attention only for himself."

Better that McMillan had ended the book there. But she has the Ultimate Answer. "President Knnedy died because he had, as man and symbol, become so many things to so many men."