Farley Mowat, the popular Canadian nature writer who was refused entry into the United States last spring, has written a cuttingly humorous book about the affair, "My Discovery of America," that has become a major seller in Canada this fall.

During their meeting last month, Canada's foreign minister, Joe Clark, presented Secretary of State George P. Shultz with a copy and suggested he read the slender volume, which features a cartoonist's sketch of a woebegone Mowat on the cover, naked except for a maple leaf, the Canadian national symbol, in place of a fig leaf. On the protruding belly of the caricature is a big stamp saying, "U.S. Immigration and Naturalization REJECT."

The Immigration and Naturalization Service action barring the popular and widely acclaimed writer from coming to the United States for a lecture and book promotion tour last April, on unspecified "ideological" grounds, touched off controversy on both sides of the border after Mowat waged a hard-hitting counterattack in the press and on U.S. and Canadian television, which he richly details in the new book.

The INS, clearly eager to close the books on the matter, subsequently agreed to let him in on special "parole" status and promised to review his files with him before determining whether the 64-year-old writer should still be included among the 50,000 persons worldwide in the service's "lookout book" who are considered inadmissible to the United States under sections of the 1952 McCarran-Walter Act. The controversial law bars individuals who would "endanger the welfare, safety or security" of the United States or have been listed as Communists, anarchists or spies.

But Mowat, during a recent interview at his early 19th-century frame Gothic-style home in the picturesque town of Port Hope, about 100 miles north of Toronto, repeated his vehement refusal of that offer, saying, "There's no goddam way I'm going to enter until I'm allowed to enter without reservations.

"To tell you the truth, I don't really give a damn. I have no great yearning to cross the great undefended border."

That is a slight compromise from his earlier position that he would go to the United States only if he received a formal, public apology and President Reagan sent up Air Force One to fly him in.

Brook Holmes, U.S. consul general in Ottawa, describes the situation now as a classic standoff.

"The offer still stands for him to go to Buffalo and discuss this," Holmes said, "but until he does so, I don't think there's going to be any movement." Just how Mowat's name ended up in the "lookout book," why it is there and when it was entered are all matters the immigration service has refused to disclose fully, even in its communications with Mowat.

INS officials did mail him what they described as a portion of his file, which consisted of copies of eight newspaper clips, all dating back to the 1960s, that detailed his activities in the Fair Play for Cuba Committee and Canadian nuclear disarmament organizations as well as his whimsical "Mouse That Roared"-style crusade stories against U.S. bomber bases. Although Mowat denies the story, he is quoted in one article as saying he once fired at a U.S. jet from his back yard with a .22-caliber rifle.

In the United States, the Mowat affair was viewed largely as an infringement on civil liberties. It created a mild uproar, causing many newspapers and some members of Congress to call for the repeal of the sections of the McCarran-Walter Act under which he was denied entry.

In the Canadian press, the refusal to admit the national folk hero, adventurer and author of 28 books, including "Never Cry Wolf," was taken as something of a national affront that was especially ironic because it came as Clark and Prime Minister Brian Mulroney were moving to strengthen trade and foreign policy with the United States.

Mowat said he wrote the new book, which his publishers said sold out its entire first printing of more than 20,000 copies, because he wanted Canadians to digest the lessons of what had happened to him.

"My rationale was I wanted Canadians to realize that if we go to bed with you, we're going to end up under it," Mowat said in the interview.

"I'm really resisting this forced marriage that Mulroney wants to create. I think it's just hastening the day."

Mowat enthusiastically acknowledged that "bureaucrat bashing," as he put it, has been one of his avocations. He has tangled with the Canadian civil service, the Hudson Bay Co. and missionaries in the Arctic among many others throughout a long and colorful career.

"I just refuse to take official structures seriously," he said, explaining why he has fought the Immigration and Naturalization Service with such apparent relish. "Most Canadians would have knuckled under without a moment's hesitation."

"Every country has its hell raisers," he said, laughing heartily, his blue eyes twinkling. "I'm part of the leavening of the rather passive national nature."

Asked about the Mowat case, Duke Austin, a spokesman for the INS in Washington, responded in an unmistakably exasperated tone.

"There's an assumption that just because he writes books about wildlife, he's all right," Austin said.

"If there are allegations about someone that are untrue or false but you refuse to come forward, how are they ever going to be cleared up?"

"He doesn't want to comply with the law of the United States in any manner," Austin added. "I kind of feel that he relishes the fact that he was denied.

"Farley Mowat would have been admitted to the United States if he had come forward," Austin said. "I'm 99 and 44 one-hundredths percent sure of that."

Immigration officials said there are about 50,000 names in the "lookout book," including about 2,000 Canadians. The general counsel of a western Canadian province and a 90-year-old artist are among them. Until he became prime minister, Pierre Trudeau was also on the list, according to a U.S. official.

The official, who requested anonymity, said many of the Canadian names were provided to the U.S. agency by Royal Canadian Mounted Police officials during the 1950s and 1960s.

In his new book, Mowat said he believes that is why he is on the list.

The U.S. official said, "I believe that would be a very good guess."

During the past three years, the U.S. official said, the Mounties have asked the INS to remove all the names they provided from the "lookout book." But the official said this is difficult, if not impossible, as the Mounties now say they no longer have any copy of their original list. The U.S. official said American authorities have no way of separating those names from the drug dealers and criminals who have been mixed in with them.

Austin said the immigration service has been unfairly the butt of criticism in the Mowat matter.

"It's not INS policy," he said. "It's the law.

"It's laid onto the bureaucracy to change a law that everybody says is crazy and wants to be changed," Austin went on heatedly. "What would you have the immigration service do? Ignore the law although Congress has not moved to change it?"

Rep. Barney Frank (D-Mass.) said in a telephone interview that he has collected 59 cosponsors of a bill he has introduced that he thinks may come up for a vote next year.

"If my bill became law, [Mowat] could reapply and not be subjected to a lot of degrading 'thought police' activity," Frank said.

Meanwhile, Mowat says he is no longer very concerned about visiting the United States and considers it an empire past its prime, disintegrating internally and relying on brute force to get its way.

"The days of the Great American Dream are over," he pronounced flatly. "Who believes in your country? Does anybody?" His interests now are not south of the border but north to the Arctic. He has teamed up with a group of Canadian filmmakers to produce a series of films on the people who live in the Far North, including Siberians and Laplanders.

Holmes, the U.S. consul general in Ottawa, wonders whether Alaska will be part of Mowat's film project and whether a desire by the naturalist to go there will ultimately end the standoff.