SAGAPONACK, N.Y. -- "You didn't ask a man hard questions, not in the Ten Thousand Islands, not in them days," says a character in Peter Matthiessen's Killing Mister Watson, his long-awaited novel about a murder in the 1910 Florida outback.

The same goes for Matthiessen himself. After a bare minimum of interviews early this summer -- in all of which, for whatever reason, he remained largely invisible -- the author has taken to refusing requests to talk about his new book.

This reticence is no recent thing. In 20 books, he's never done a publicity tour; he hardly ever goes on television; he accepts only the occasional speaking engagement. He is one of the few first-rate modern writers who have never sat for a Paris Review interview -- and the reason is not just that he was one of the magazine's two founders.

But Matthiessen can be pressed to talk about the issues that concern him deeply, which is why he sat answering a few questions in his back yard here last month as people bustled around preparing for a benefit. This wasn't the typical Hamptons book party: For one thing, the central figure is serving consecutive life sentences at the federal penitentiary in Leavenworth, Kans.

This is a tangled web, involving platoons of lawyers, a book that was suppressed by its publisher, members of the U.S. Congress and Canadian Parliament, millions of Soviet citizens, two of the longest and most expensive libel trials in U.S. history, and a time only 15 years ago that already seems unimaginably distant, when a state of near civil war existed between radical Indian groups and the government.

The precipitating event for all this was a 1975 gun battle on the Pine Ridge Reservation in South Dakota between two FBI agents and members of the American Indian Movement (AIM), a firefight that resulted in the deaths of the agents and one Indian. It's never been established who fired the first shot.

Leonard Peltier, an Ojibwa-Sioux AIM activist who is now 45 years old, was convicted of murdering the agents, who were finished off at close range after being wounded. While working on a book titled Indian Country, Matthiessen became convinced of Peltier's innocence. His 1983 work, In the Spirit of Crazy Horse, is an impassioned, partisan examination of the situation of Native Americans in general and of the Peltier case in particular.

It's a book more famous than read, because soon after publication Matthiessen was sued for libel by William J. Janklow, then governor of South Dakota (for $24 million), and an FBI agent involved in the case (for $25 million). Viking responded by destroying the copies of the book it had in the warehouse; it's since been unavailable except as a pricey item on the rare-book market. Nevertheless, it's proven influential. Direct evidence of that was available at the benefit in the form of filmmaker Suzie Baer.

While an undergraduate at New York University's School of Visual Arts, Baer got a copy of Crazy Horse from a friend who had bought it for a bargain $10 from a sidewalk peddler. She decided to devote her senior thesis to Peltier's case, a brief film now in the process of mushrooming into an hour-long documentary.

"The more I learned about Indian issues," she said, "the bigger the story became." Admission to the benefit cost $75, which will go to funding the film; a quarter of any profits from the completed work, in turn, will go to Peltier's defense fund.

Thirty years ago, when Matthiessen moved out to this end of Long Island, the only local citizens available for a benefit would have been potato farmers. Nowadays, you can't stroll down a lane in the Hamptons without being beaned by a foul ball from a celebrity softball game, a fact that proved handy in compiling the guest list. It didn't hurt, either, that Killing Mister Watson was on national bestseller lists.

Despite this, Matthiessen expressed some trepidation. "I'm not into benefits. It's not my style, and frankly I've been dreading it for two weeks." For one thing, he said he was worried about his tendency to mumble in public. And then there was the likelihood of rain, which tends to be disastrous for garden parties.

None of these fears came to pass. About a hundred guests paid at least the minimum to attend and eat watercress sandwiches, fruit salad, oysters and assorted other canapes prepared by the local Shinnecock Indian tribe. A video monitor showed the 13-minute completed portion of Baer's film, the AIM song played over loudspeakers and various celebrities -- including E.L. Doctorow, Kurt Vonnegut, John Irving -- mingled.

Pacing a bit, arms crossed over his T-shirt, Matthiessen began his short speech by telling the crowd that Peltier assured him in their first meeting of his innocence. "I had every reason to believe he was wrongly accused. Now I know he was wrongly accused," he continued.

A few months ago, he said, he interviewed a hooded man on the West Coast who had an Indian-style braid below his shoulders. "I spoke to him for five hours. This is the man who killed the agents. He has now come forward, and feels badly that Leonard has done 15 years so far. However, he does not feel that he did anything wrong. He killed those men in self-defense. The U.S. military came in to the reservation with very aggressive tactics."

Two days previously, Matthiessen said, he had a phone call from South Dakota. The ex-governor's claims of libel had been thrown out by the state Supreme Court. (Earlier this year, the FBI agent's final appeal had been rejected by the U.S. Supreme Court.)

The 4-1 decision was strongly in favor of Matthiessen and his publisher, noting not only that Janklow was a public official when the book came out and thus difficult to libel, but that the statements in question "are amply supported by credible, reliable sources." Among the disputed allegations: that Janklow raped a 15-year-old Indian girl in 1967 while he was director of her tribe's Legal Services program. GRADUALLY, the Peltier case has become a flashpoint of sorts. To his defenders, Peltier is a home-grown Mandela; to the Soviets, for whom he has an obvious propaganda value, he is as famous and important as Sakharov was in this country. Millions of Soviets signed a petition for him that was delivered to the White House. Even Hollywood, in the form of director Oliver Stone, has gotten interested. Stone plans a film on the case.

Peltier and his supporters say he was imprisoned because someone had to pay for the murder of the agents. After an earlier case against two of the Pine Ridge Indians had resulted in their acquittal, Matthiessen writes in Crazy Horse, Peltier "represented the FBI's last chance to obtain a conviction."

What is most convincing to Peltier's supporters are the questions that have been raised about the government's handling of the evidence and the trial. For example, Canada agreed to extradite Peltier largely on the basis of the one witness who said she had seen him kill the agents. The witness was a disturbed woman who later recanted, saying her testimony had been coerced.

On the "West 57th" television show last year, the prosecutor in the case, Lynn Crooks, said: "I don't agree we did anything wrong with that, but I can tell you it don't bother my conscience if we did." Statements like these tend not to please the Canadians, who take the matter of extradition seriously.

And then there's the matter of a shell casing found near the agents' bodies that the government identified as coming from Peltier's gun -- except, it came out after the trial, it really didn't. As an appeals judge in the case told "West 57th": "The FBI withheld from the defense a good deal of ballistic information which might have been helpful."

This is one reason nearly 50 members of Congress asked that Peltier, who has exhausted his appeals, be given a new trial. At the moment, though, said Rep. Don Edwards (D-Calif.), "There is a standstill. There's nothing Congress can do, and the Department of Justice would fight it very hard, what with the alleged murder of two FBI agents."

Sen. Daniel Inouye (D-Hawaii) is more optimistic. He became interested in the case after seeing the "West 57th" segment last year. He was particularly upset by the bit about the shell casing.

"I was surprised and in a way shocked to hear the prosecutor say that throughout the trial he held his breath because he had some information which, if asked, he would have to disclose," said Inouye. "I've always believed a prosecutor's job is not to get the defendant, but to make sure justice is carried out. If you know the person is innocent, you shouldn't hide that fact."

Inouye said he is pursuing the only option available to Peltier: a pardon by President Bush. (Oddly, the president and Matthiessen are linked: Their sisters roomed together in college.) While a pardon may not seem likely, Inouye said, "You can never tell without inquiring." AFTER HIS speech, Matthiessen asked a friend: "Do I have a future as a rabble rouser?"

"You have a present and a past as a rabble-rouser," she assured him.

In the Spirit of Crazy Horse, a new edition of which will be on tap as soon as Viking is convinced the lawsuits are completely over, may be Matthiessen's most political effort, but throughout his career he's shown a dedication to giving voice to the voiceless.

His best-known novel, At Play in the Fields of the Lord (1965), currently being filmed by Hector Babenco, puts missionaries and a soldier of fortune in conflict over an Amazon tribe; Far Tortuga, published a decade later, uses the style of a prose poem to depict the doomed voyage of a group of Caribbean fishermen. And now, in Killing Mister Watson, he's obviously as interested in the swampland setting and the characters who live there as in the story.

It's a method that seems to have some appeal. There have been one or two dissenting voices, but generally the book has received an enthusiastic welcome. "I'm rather surprised," the author said. "I thought a book about a turn-of-the-century, not-very-promising human being . . . It doesn't seem like a very popular subject."

Known as a writer who never returns to the same territory twice, Matthiessen surprisingly said there will be at least one more Watson book. In fact, it is likely to become a trilogy.

The second time, "I'll be dealing with Watson more from the point of view of trying to figure out what really happened and who was Leslie Cox -- the guy who did the actual killing -- and what became of him, and what became of Watson's son, and what was the effect on Watson's family." The new novel, he promised, won't seem redundant to readers of the first effort.

There are connections between the Watson books and In the Spirit of Crazy Horse: both concern the ambiguity that surrounds a murder and the competing voices and claims about the victims and the accused. The author, however, doesn't see such a link. "Only that there's Indian people in both," he said. "They've always been in the shadow of my works."