YURI IVANOVICH NOSENKO had endured far more arduous interrogations. This one lasted only four hours and it was not held in the padded basement room where the Central Intelligence Agency had once confined him for three long years. Now drawing a $30,000-a-year allowance from that same CIA, Nosenko presented himself on a March afternoon in 1976 at the Washington offices of Reader's Digest. His interviewer, Edward Jay Epstein, concluded the questioning that evening with a flourish: dinner at an elegant French restaurant a couple of blocks away.

That the interview took place at all was remarkable. Nosenko is a former KGB officer who defected to the United States just 10 weeks after the assassination of President John F. Kennedy. According to the CIA, exactly what he had to say is still so sensitive, so special, so secret that its disclosure even now could "only interfere with American counterintelligence efforts." Yet according to Epstein, who tape-recorded Nosenko's remarks for this book, "the CIA put me onto him."

No doubt the CIA thought it would get a good press. "I presume that it found out I was writing a book on Lee Harvey Oswald and it wanted me to put Nosenko's message in it," Epstein told New York magazine recently. "Nosenko's message was that Oswald was a complete loner in the Soviety Union and never had any connection or debriefing by the KGB."

Epstein then began talking to the Agency's formidable ex-chief of counterintelligence, James Jesus Angleton. He had a darker view of Nosenko's pressence in this country.

What Epstein has written, hundreds of interviews later, is a fascinating, important and essentially dishonest book. Fascinating because it offers new information about Oswald, about the KGB, and about the CIA. Dishonest because it pretends to be objective, because it is saddled with demonstrable errors and inexcusable omissions, because it assumes that the KGB always knows what it is doing while the CIA does not. It is paranoid. It is naive.

Nosenko's defection was officially proclaimed by the State Department on Feb. 9, 1964, whereupon he quickly disappeared from public notice. He told the FBI that he had personally supervised the KGB's file on Lee Harvey Oswald and thus could assure the Americans that Oswald had no connection with the KGB.

Epstein concludes, as Angleton obviously had, that "Nosenko was a Soviet intelligence agent dispatched by the KBG expressly for the purpose of delivering disinformaton to the CIA, FBI and Warren Commission."

In short, Lee Harvey Oswald, the supposed lone assassin of President Kennedy, may well have been working for the KBG at one point or another in his shabby life. Nosenko said this wasn't true. And therefore, according to Legend's logic, it was. Oswald, the ex-Marine who had defected to Russia in 1959 and returned three years later, had been living a "legend," a false biography concocted for him by the KBG.

That is far from the most startling assertion that Epstein has to make. Legend is really two books, stretched thin. His central message, although cushioned with all the careful ambiguities of a State Department communique, is that the highest echelons of the American intelligence community have been infiltrated by the KBG, penetrated by an enemy "mole" who made his way to some key position at the CIA or some other agency.

It is all quite plausible. The British and West German intelligence services had been successfully compromised by the Soviets since World War II. Kim Philby, who was recruited at his university, rose to become the head of the counterintelligence division of Britain's MI-5 before he was exposed. In West Germany, Epstein notes, the Soviets succeeded in the getting their own man, Heinz Felfer, installed as head of counterintelligence by sacrificing other agents "like pawns in a chess game." So why not here? The metaphysics of espionage, where nothing is what it seems, can be seductive. Judging from Epstein's book, the best proof of the existence of an American "nole" lies in the fact that he hasn't been found yet. Another piece of evidence: Nosenko told the CIA there was no "Mr. Big." Step up the search!

Surprisingly, Legend is weakest where it should be strongest, demonstrably slipshod where it should be solid. Epstein's first book, Inquest: The Warren Commission and the Establishment of Truth, was one of the first to expose the shortcomings of that inquiry. Yet here he deals with the Kennedy assassination in a cavalier appendix entitled "The Status of the Evidence" that makes one wonder whether Epstein has even glanced at the Warren Report in the last 10 years. He seems not to have even looked at the pictures.

Take, for example, Epstein's confident assertion that the Warren Commission "made a serious error in reckoning the elapsed time" from the first rifle shot to the last. The Commission, he declares, staged a reconstruction of the assassination in mid-1964 when the oak tree blocking the line of sight from the sixth floor of the Texas School Book Depository "was in full bloom. But the assassination occurred on November 22nd when the deciduous tree had no foliage." Therefore, the assassin had more time to fire than the Commission gave him.

It sounds like a nifty piece of detective work on the part of Edward Jay Epstein. But wait a minute. No foliage? There were plenty of leaves on the live oak (an evergreen) that AP photographer James W. Altgens captured at the top of his picture showing the President of the United States being hit by a bullet on Nov. 22, 1963. The photograph can be found in any copy of the Warren Report on page 113.

This is far from the only shortcoming. The footnotes are too sparse, the documentation is fuzzy, and occasionally even the datges Epstein cites are just plain wrong. For a project financed by Reader's DIgest, reportedly for $500,000, the reader has a right to better scholarship - and to more information. Epstein tells more in his promotional interviews about the book than he does in the book itself.

He assured New York magazine, for instance, that he really doesn't think the Russians were involed in JFK's assassination. "I think that the fact that Oswald traces so clearly back to the Russians makes it extremely unlikely that they would have recruited him as an assassin," Epstein was quoted as saying in the magazine's March 6 issue.

Epstein does write, in an early chapter, that "Neither Angleton's shop nor the CIA's Soviet Russia division believed that Oswald was acting under the control of Soviet intelligence when he assassinated the president. (In fact, circumstantial evidence seemed to diminish that possibility.) It seems far more likely to both that the relationship Nosenko was attempting to protect might be a prior connection Oswald had had with the KGB." That said, Legend marches on conspiratorially to Nov. 22, 1963 in a chapter called "The Day of the Assassin," which is the concluding segment of a section subtly titled "The Mission." The book is full of subliminal messages that Epstein avoids stating openly. What, for instance, are we to think of all those bungled assassination plots against Fidel Castro when they have been hatched in a CIA compromised by a high-ranking enemy "mole"?

Unfortunately, Legend has a pervasive weakness, a persistent double standard. It keeps assigning omniscience to every Soviet move and Deliberate intent to every omission. But what the American intelligence agencies do and say is usually kissed off in a footnote or mentioned only in passing. Epstein does not even mention, much less deal with, Nosenko's report to the FBI that the KGB not only had no connection with Oswald, but also suspected him of being an American "sleeper" agent.

And what of Epstein's perhaps unwitting disclosures - in the book and in New York magazine - that Angleton's counterintelligence experts had intercepted a stridently anti-American letter Oswald wrote to his brother in 1959 and another in which Oswald said he had seen U-2 pilot Francis Gary Powers in Moscow. What's going on here? As late as August 10, 1976, CIA Director George Bush assured a House subcommittee that "the only correspondence to or from Oswald that was intercepted was one letter, dated 8 July 1961, to Mr. and Mrs Lee Harvey Oswald, from his mother . . ." Perversely, for all its shortcomings, Legend commands serious attention. It is, as one of the publisher's blurbs states, "a sensational, highly controversial expose," drawn from a storehouse of declassified documents, including some obtained only by Epstein, and interviews with more than 400 people, many of them not interviewed by the Warren Commission. It throws new light on Oswald's life, especially in Japan where he apparently dated a nightclub hostess who cost more than his take-home pay and where he reportedly "became involved with a small circle of Japanese communists."

The freshest revelations, however, are those about Nosenko. That they came from Angleton and like-minded colleagues makes them all the more intriguing. What former CIA Director William E. Colby has described as Angelton's "ultraconspiratorial" view of the world is apparently no longer in vogue at the agency. But if his theories were doubted (Colby, for one, believed they did the CIA more harm than good), his brilliance never was. Even today, no one in the intelligence community seems brash enough to assert that Angleton didn't know what he was talking about. He seems to have kept too many secrets to himself, hoarding them like ammunition. In any case, professional disagreement with the CIA's chief of counterintelligence was always cautiously stated.

In his own forthcoming book, Honorable Men: My Life in the CIA, Colby puts it this way:

"I spent several long sessions doing my best to follow his torturous conspiracy theories about the long arm of a powerful and wily KBG at work, over decades, placing its agents in the heart of allied and neutral nations and sending its false defectors to influence and undermine American policy.I confess that I couldn't quite absorb it, possibly because I did not have the requisite grasp of this labyrinthine subject, possibly because Angleton's explanations were impossible to follow, or possibly because the evidence just didn't add up to his conclusions; and I finally concluded that the last was the only real answer. At the same time, I looked in vain for some tangible results in the counterintelligence field, and found little or none. I did not suspect Angleton and his staff of engaging in improper activities. I just could not figure out what they were doing at all."

Nonetheless, Angleton's suspicions about Nosenko - at least as reported by Epstein - cannot be easily dismissed. The Russian KBG officer first surfaced as a CIA informant in 1962, just six months after another Soviet intelligence officer, Anatoli M. Golitsin (code name: Stone), had defected with the starting report that a high-ranking "mole" had already been planted in the American system. Nosenko, in effect, assured the CIA that the "mole" was no more than a mouse, a low-ranking American military man who once worked as a motor pool mechanic at the U.S. Embassy in Moscow.

Nosenko's own defection in February of 1964, with his claims to full knowledge of the KGB case file on Oswald, led Angleton and other CIA skeptics to the discovery of one inconsistency after another. But FBI Director J. Edgar wasn't interested. According to Epstein, Hoover was more concerned about covering up the FBI's failure to keep a closer watch on Oswald before the assassination. "By an odd twist of fate, the FBI's interest lay in concealing, rather than revealing, any hint of Soviet involvement," Epstein writes.

The infighting was evidently fierce. By the spring of 1964, apparently on the heels of two FBI interviews that took Nosenko at his word, the CIA, reportedly with the approval of Attorney General Robert F. Kennedy, put Nosenko in solitary confinement and began a grueling "hostile interrogation" in hopes that the KGB man would break down before the Warrent Commission had to submit its report.

The ploy didn't work. The Warren Commission decided not to question Nosenko at all, ostensibly following a June 24, 1964, conference between Warren and the CIA's Richard Helms. Helms told the chief justice that it was still unclear whether Nosenko was a legitimate defector or a Soviet disinformation agent.

The only trouble with that sequence is that the Commission took up the question of Nosenko the day before, on June 23, 1964. Could it have decided to call Nosenko, only to have Helms head off the showdown by buttonholing Warren the next morning? No one knows. The CIA has thus far steadfastly refused to let the transcript be made public - on the mind-boggling grounds that the release of any information about Nosenko "can only interfere with American counterintelligence efforts."

The CIA kept hammering away at Nosenko, keep him in custody without any legal or constitutional authority until 1967. His disbelievers in the CIA's Soviet Russia division compiled a 900-page report, chronicling all the information he had provided. It concluded that he was a fake, assigned by the KBG to mislead the investigators of President Kennedy's assassination. But Nosenko had his defenders, too, and they finally prevailed with a 500-page reply that won its author the CIA intelligence medal. For Nosenko, who is reputedly under a death sentence in Mother Russia, the Agency provided a $30,000-a-year allowance, a new identity and a new home. Six years later, Epstein writes in a simplistic version of the event, Angleton was forced into retirement by Colby on the eve of The New York Times' disclosure of illicit domestic activities at the agency. Angleton's top aides were forced out with him. The new counterintelligence crowd appointed Nosenko one of its consultants.

Epstein's conclusion is ominous: "With Nosenko accredited and the counterintelligence staff purged, the CIA had truly been turned inside out."

Oversimplified? Of course. Overstated? Absolutely. Some truth to the book? Undoubtedly. Where? Who knows? But watch out for those oak trees.