THE PALACE FILE By Nguyen Tien Hung and Jerrold L. Schecter. Illustrated. 542 pp. New York: Harper & Row.
WRITTEN largely from a Saigon perspective, this book provides significant new data on United States relations with South Vietnam from 1968 to 1975 and fresh insights into the character of Nguyen Van Thieu, the general whom the United States supported as South Vietnam's President from 1967 to 1975. An extraordinary collection of previously undisclosed exchanges of letters between Mr. Thieu and Presidents Nixon and Ford combine with the authors' interviews with many key Americans and Vietnamese to make this an important study.
Nguyen Tien Hung, an American-trained economist who currently teaches at Howard University, was with the International Monetary Fund before being appointed special assistant to President Thieu in 1973. He later became Minister of Economic Development and Planning and one of Mr. Thieu's closest advisers. Jerrold Schecter, author of an earlier book on Vietnam, ''The New Face of Buddha,'' covered Vietnam for Time magazine in the early 1960's; from 1971 to 1975 he was the magazine's White House correspondent and then its diplomatic editor.

The authors of ''The Palace File'' establish that a reluctant Mr. Thieu was induced to support the 1973 Paris accords, which for Americans ended the Vietnam War, because of Mr. Nixon's repeated private promises that the United States would punish North Vietnam with heavy bombing if it significantly violated the agreements, and by his clear threat that American economic support might end if the South Vietnamese President refused to sign them. ''For President Thieu and his government, the real agreement in Paris was not what had been signed . . . with North Vietnam and the Provisional Revolutionary Government, but the private promises Nixon had made in his letters to Thieu ever since December 1971.'' Mr. Nixon's commitment, and Mr. Ford's somewhat less explicit private assurances, led Mr. Thieu to believe that despite the mounting antiwar mood in Congress the White House could still be counted on to back up the Paris accords with United States air and naval power.
The authors persuasively present a charge of duplicity, directed primarily against former Secretary of State Henry Kissinger, but also implicating Mr. Nixon and ultimately Mr. Ford, who, with his eye on the 1976 election, shied away from greater assistance to Mr. Thieu for fear of ''making it his war instead of Nixon's.'' Mr. Thieu was especially bitter toward Mr. Kissinger: ''If Kissinger had the power to bomb the Independence Palace to force me to sign the [ Paris ] agreement, he would not hesitate to do so.''
Mr. Thieu had assumed Mr. Nixon was in his debt for his help in getting Mr. Nixon elected in 1968, for he had refused President Johnson's request to participate in negotiations with Hanoi, thereby undercutting Vice President Humphrey's efforts to present himself as a man of peace who could end the war through negotiation. The authors show that Mr. Thieu made a major mistake in his reliance on Mr. Nixon and Mr. Ford and in his fear that he would antagonize them if he released their letters, incorporating their firm commitments, to Congress and the American public, even after Hanoi had begun its 1975 offensive. But they do not provide the facts to warrant their assertion: ''The weight of the evidence shows that had the United States kept the promises made in writing to Thieu to convince him to sign the Paris Accords, the Republic of South Vietnam could have survived.''
Even if after 1973 Congress had restored earlier levels of economic support and military supplies and agreed to punitive United States bombing of the north, it is doubtful Mr. Thieu's regime could have been shored up much longer. There was too much wrong with it. Sotto voce, the authors acknowledge its weakness - an ineffective, repressive government artificially sustained from the outset by massive American economic support and military supplies, honeycombed with corruption and led by a man whose popular support was so weak that he perceived his ''mandate from heaven'' as issuing predominantly from Washington. Having earlier helped overthrow Ngo Dinh Diem in an American-supported coup, Mr. Thieu constantly worried that his fate might be similar if Washington became dissatisfied with his own stewardship. THE book treats the Washington-Hanoi dimension of the Paris accords and the negotiations leading up to them less comprehensively than it treats the Washington-Saigon level, and there are some serious inaccuracies in this treatment. For example, the authors write, ''Hanoi scorned aid offered by the U.S. to 'heal the wounds of war.' '' Indeed, in order to consummate the negotiations, Mr. Nixon felt obliged to offer Hanoi $3.25 billion in reconstruction aid, and that offer - though never implemented - was certainly not scorned.
As for the 1973 accords, the authors conclude: ''Although Kissinger made it appear that the North Vietnamese had made concessions, in fact it was the U.S. which totally backed down on all major issues.'' As one of his aides referred to the Christmas bombings: ''We bombed them into accepting our concessions.'' And what did the Nixon Administration actually accomplish? ''During the nearly four years Kissinger negotiated for an agreement, the bloodshed mounted: 15,000 Americans were killed and 100,000 Americans wounded while hundreds of thousands of Vietnamese lives were lost.'' In the end, Mr. Kissinger ''arrived at an agreement that was essentially the same as the one the Communists presented to him in May 1969.'' But he had secured his ''decent interval,'' and Mr. Nixon found it easier to sustain his claim of achieving ''peace with honor.''