David A. Stockman's new book -- damning as its description of the Reagan administration may be -- is unlikely to have much impact on Ronald Reagan's reputation, but could significantly alter David Stockman's.

Reagan's historical fate will be determined by the ultimate impact of his presidency, not by the memoirs of his associates. But judging by the excerpts that Newsweek magazine released yesterday, Stockman's book could alter public perceptions of the former director of the Office of Management and Budget, not necessarily in ways Stockman envisioned when he sat down to write "The Triumph of Politics."

Until now, Stockman has been known as the intellectual wunderkind of the Reagan White House, the ultimate numbers-cruncher and chief designer of the "Reagan revolution" who erred mostly by being too frank with journalist William Greider, who published Stockman's indiscretions in the Atlantic Monthly.

The book provides a richer, more complex portrait of a confessed ideological zealot and political naif who deceived himself, his colleagues and his president, and who now has no compunction about describing himself and the administration he served in devastating terms.

Stockman writes that he was a victim of "ideological hubris" who made the mistake of trying to "peer deep into the veil of the future and chain the ship of state to an exacting blueprint" -- his description now of the true import of the Reagan revolution. But a radical reordering of national priorities based on an ideological conception "can't be done," Stockman writes in the book. "It shouldn't have been tried."

In the Reagan White House, he writes, Stockman was virtually alone in understanding the complex issues of fiscal policy and taxes. The "California crowd" of longtime Reagan advisers "were illiterate when it came to the essential equation of policy" and tended to care only about how the administration looked on the network news each night.

"I could have performed a public service" by quitting the government in late 1981 and telling the truth about the White House's deception of Congress and the country, he writes. Instead he chose to stick it out as an insider. His original quest to radically shrink the federal government, Stockman now acknowledges, was a pipe dream. "I wasn't wise enough," he told Newsweek in an interview last week. "I had been in Congress four years [as a representative from Michigan, after six years as a congressional aide], but I didn't learn anything about politics."

In the book Stockman says that his original plan would have meant "complete elimination of subsidies to farmers and businesses. It required an immediate end to welfare for the ablebodied poor. It meant no right to draw more from the Social Security fund than retirees had actually contributed, which was a lot less than most were currently getting . . . . The world could indeed be started anew . . . . "

In hindsight, Stockman writes, that was much too much to ask. "Only an iron chancellor would have tried to make it stick. Reagan wasn't that by a long shot."

If the first Newsweek excerpt is a reliable guide, Stockman's book is riddled with mea culpas. His early assurances that he knew the answers to all the big fiscal questions were misleading: "Only later would I appreciate the vast web of confusion and self-delusion I was creating. I instilled so much confidence by appearing to know all the answers, but I was just beginning to understand the true complexities of the federal budget."

Reagan, Stockman writes, never confronted the fact that there was no money to pay for hundreds of billions of dollars in spending and tax cuts he envisioned. But Reagan should not be blamed for this oversight, Stockman writes: "I never provided him with a single briefing on this."

When, during the first week in office, it became clear to Stockman that the new administration's economic plan could add $600 billion to the federal deficit over five years, he evaded the implications of his realization. "I should have blown the whistle and called off the blitzkrieg [to launch Reagan's new economic policies]. But a radical ideologue at the height of his powers does not stop, in mid-headlong rush, to wonder how history will judge him years from then. I had momentum, I had won victories, I would win more. The pace couldn't be slackened for a moment . . . . I soon became a veritable incubator of shortcuts, schemes and devices to overcome the truth now upon us -- that the budget gap couldn't be closed except by a dictator."

Some readers of Stockman's book may wonder about the man's sense of political loyalty. Stockman addresses the subject in his discussion of his indiscreet interviews with Greider. White House aides "missed the target when they went for my scalp" after Greider revealed Stockman's grave doubts about the plausibility of Reaganomics. "They thought loyalty to Ronald Reagan required drastic action to nip a bad story in the bud. But it was petty loyalty that amounted to profound disloyalty in the larger scheme of things."

In the end Stockman says that his ideological conception of how government should be changed was wrong. The country really wants a welfare state, not his stripped-down federal government, he writes. Congress actually does represent its constituents' desires.

But all the mea culpas do not lead to any fundamental confession from Stockman. In last week's interview with Newsweek, he acknowledged errors, but not wrongdoing. "Every man is entitled to be a crusader. And if he has a vision that says society is going to be better off if you implement this doctrine -- go to it hammer and tongs. And so in a way I don't think what I did was wrong. On the other hand, I have to judge that it wasn't realistic."