TWENTY YEARS HAVE passed since the assassina-

tion of President John F. Kennedy, and there must be millions upon millions of Americans who don't know much about it, and even less about the man accused of the murder: Lee Harvey Oswald.

Now comes a new book dedicated to an old proposition: Oswald did it and he did it alone. The theme of Oswald's Game by Jean Davison is that he was a disturbed young radical who decided to kill Kennedy after reading Cuban Premier Fidel Castro's pointedly public complaints in September 1963 about attempts on the lives of Cuban leaders. It is, as Norman Mailer observes in a curious foreword, "the Warren Commission revisited." Apparently, there's no news like old news.

Davison tells a good story. And she aims some well- deserved arrows at the distortions of conspiracy theorists like Mark Lane (Rush to Judgment) and the absurdities of such other students of the crime as Michael Eddowes (The Oswald File) and David Lifton (Best Evidence). But she uses the straw-man technique. She really doesn't address the question of whether a conspiracy was involved. Her "selected bibliography" ignores some of the most trenchant books on the subject and, aside from the 26 volumes issued by the Warren Commission and subsequent Congressional studies, she appears to rely almost entirely on secondary sources. She shows hardly any familiarity with the hundreds of thousands of pages of FBI and CIA documents that have become available over the years at the National Archives and elsewhere. Oswald's Game contains nothing that is new.

As a primer on Oswald, however, the book is a succinct reminder of what a swinish fellow he was, a conniving, arrogant, self-centered delinquent who pulled a knife on his half- brother's wife at age 12 and was hauled before a juvenile court judge in New York at age 13. A few weeks later, he happened to be handed a pamphlet urging clemency for Julius and Ethel Rosenberg, then awaiting execution. The pamphlet, he said, stuck in his mind for years.

From New York, his mother Marguerite, a "monstrously self-centered" person who had Oswald's father buried the very afternoon he died in 1939 (two months before Oswald was born), took Lee to New Orleans. He joined the Civil Air Patrol and got a job at a dental lab where a teen-age co- worker found him "very serious about the virtues of Communism." A friend, Palmer McBride, remembered how Oswald once told him "that he would like to kill President Eisenhower because he was exploiting the working class."

From New Orleans he went to Texas, to the Marine Corps, and then defected to Russia where he threatened, on a visit to the U.S. Embassy, to tell Soviet officials about his work as a radar operator. Disenchanted with Russia, he came back to the United States where, Davison submits, he remained a Marxist, disillusioned by the Russian Revolution but still enthralled by Castro and anxious "above all . . . to help Cuba."

In April 1963, Oswald fired a rifle shot through a window one night at former Major General Edwin A. Walker, a rightwing enthusiast who had been advocating the invasion of Cuba. The bullet just missed. Some two weeks later Oswald moved back to New Orleans to look for work. He was still there in early September when Castro gave a spur-of-the- moment, three-hour interview to Associated Press reporter Daniel Harker during a reception at the Brazilian Embassy in Havana, producing a story that both New Orleans papers printed:

"HAVANA (AP)--Prime Minister Fidel Castro said Saturday night 'United States leaders' would be in danger if they helped in any attempt to do away with leaders of Cuba. Bitterly denouncing what he called U.S.-prompted raids on Cuban territory, Castro said, 'We are prepared to fight them and answer in kind. United States leaders should think that if they are aiding terrorist plans to eliminate Cuban leaders, they themselves will not be safe.'"

19.9 It is Davison's belief, which is plausible enough, though not original, that Oswald read that story, concluded that the Kennedy Administration was indeed planning to kill Cuban leaders, and eventually acted on that conclusion. According to a long-suppressed (until the mid-'70s) letter J. Edgar Hoover wrote the Warren Commission, a high-level Communist Party informant (Morris Childs, code-named "Solo," although Davison inexplicably does not name him) met with Castro after the Kennedy murder. According to "Solo," Castro told him that Oswald had talked of assassinating the president on a visit to the Cuban Embassy in Mexico City in late September. In October, Oswald happened to watch a TV double-feature about assassination plots: Suddenly, starring Frank Sinatra, concerned an abortive attempt to kill an American president; and We Were Strangers, starring John Garfield, was about the overthrow of the Machado regime in Cuba in 1933. Marina Oswald told the Warren Commission he watched the films "with interest."

One would think the Garfield movie would have reminded Oswald of Batista, if anyone, but Davison reads a great deal into this incident, theorizing that Oswald saw parallels with "the American-backed plots against Castro. . . . He was excited because the double feature practically read his mind. Coincidences like this one can make almost anyone believe that fate has intervened. If I'm right, these two movies must have seemed like a tug at his sleeve."

A free-lance writer who became fascinated by the assassination in 1968, Davison says she soon discovered in her readings "that it was possible to manipulate the evidence to support any position I took (whether it was, for example, that Oswald worked for the FBI or that he did not work for the FBI). Unfortunately, whichever stand I chose, there was always evidence left over that seemed to contradict it."

Unfortunately, Davison's own handling of some of the evidence is questionable in the same way. She notes, for example, that one of Oswald's favorite TV programs as a child was I Led Three Lives, an anti-communist program "that stressed the supposedly clandestine and subversive nature of Party work." Remarkably, she concludes, "This kind of life--being an outsider and secretly fighting the authorities-would likely have appealed to him."

An outsider secretly fighting the authorities? The program was about Herbert Philbrick, who spied on communists for the FBI.

Another lapse. "Like his fellow radar operators, Oswald," we are told, "had a low security clearance." The main authority for this statement is given as former Marine Corps lieutenant John Donovan, Oswald's onetime crew chief, and Davison partially sums up his Warren Commission testimony about "the confidential information Oswald had access to."

She entirely overlooks Donovan's testimony --from the very same page she cites as her authority--that Oswald must have had at least a secret clearance "to work in the radar center because that was a minimum requirement for all of us." It may have been higher. According to Donovan, Oswald's defection also required the changing of various "codes."

Davison assures us at the end that "the assassination of John Kennedy was neither an act of random violence nor a conspiracy" but rather "a result of Oswald's character and background interacting with circumstance." It is Oswald who commands her attention, and the book amounts to a sort of psychohistory of the man and his motives.

Unfortunately, her conclusions about him are flawed by her failure to give sufficient attention to the crime itself-- as though it did not matter. She appears to take the view that the Warren Commission is right because it said it was right. She accepts what is congenial and flies past that which is not. Was there a shot from the grassy knoll, to the right of Kennedy's motorcade, while Oswald fired from the Texas School Book Depository? Davison simply shrugs off the question in two quick sentences, then adds:

"In any event, the bulk of the evidence about Oswald clearly suggests that if there had been a conspiracy, Oswald would not have been a patsy, but the ringleader."

In his foreword, Norman Mailer tells how he encouraged Davison to write her own book, in response to a letter she sent him, and then professes his disagreement with the result. Mailer suggests that the best way to look on Oswald's Game is to think of it in terms of field artillery, wherein forward observers are told to bracket a target. If the first shots, the conspiracy books, fall short, then the next shot should be targeted to land on the far side. That way, Mailer reasons, "by comparing the near and the long, they can approach a direct hit."

Mailer, it may be presumed, will get a free copy of the book. The reader may conclude that $17.95 is too much to pay for a shot that falls wide of the mark.