LAURENCE I. BARRETT, Time magazine's White House correspondent, has written a midterm appraisal of the Reagan presidency that is remarkable for its thorough research, for its penetration into the decision-making processes of the regime, for the details of its two-year chronicle of American politics, and unique in the relationship of the author with his sources.

Eschewing Walter Lippmann's celebrated advice that reporters ought to keep officials at arm's length, Barrett has managed to combine the roles of a reporter of the present, a historian of the immediate past, an analyst of a policy and a confidante. It will be surprising if there aren't a few reproaches resembling those that arose between David Stockman and William Greider in a situation where there were so many opportunities to confuse things on and off the record.

The book is reminiscent of Ernest K. Lindley's similar effort, Half Way With Roosevelt, written at FDR's midterm. Lindley was The New York Herald Tribune's correspondent in 1936 and in the Newsweek bureau in 1937. That book celebrated the Roosevelt expansion of the powers of the federal government to deal with economic and social problems; this one celebrates the Reagan effort to contract the welfare state. One dealt with the beginning of an epoch in American history; the other with an effort to end the era of federal expansion.

Lindley's book on Roosevelt had more partisanship and advocacy in it than Barrett's book on Reagan. Lindley had no doubts about what Roosevelt had achieved "half-way" through his first term; Barrett has his reservations. The critics of FDR did not much like Lindley's book; the critics of Reagan will not much like this one, even though it is a lot more objective in its estimate of the Republican president and a lot more reserved in its judgment of policies.

Barrett's first chapter is a remarkably concise and precise appraisal of Reagan so far. It faults the "overplayed hand" of the first 18 months for causing more economic trauma than was necessary. But it also describes him as "an effective President" who has made an "activist" record that will leave its impact whatever happens in the next two years. The 1982 losses of the administration among low-income and black voters in Barrett's view "creates doubt as to whether Reagan, or any Republican, could reassemble in 1984, the vote pool that gave the GOP the White House and the Senate in 1980."

The book's portraits of Reagan and the men around him are sharp and incisive. Secretary of Defense Caspar Weinberger comes off badly: "the worst secretary of defense since Louis Johnson." Chief of Staff James Baker scores high, with good personal marks for many others. The sketch of "Rawhide," the president himself, is illuminating. His essential optimism is criticized as an administrative flaw depriving Reagan of the quick recognition of both policy and personnel weakness and praised as an attribute of leadership in a democracy. "A leader," says Barrett, "must be able to evoke confidence, even courage, when objective circumstances indicate that apprehension would be the purely logical response. . . . By an accident of history, it was a necessary talent in the early 1980s. Reagan was never passive in exercising that talent."

Barrett is at pains to show that Reagan, while committed to reducing the powers of government as a policy goal, is no fanatic and possesses ability to compromise and accommodate.

Barrett, at many points, deals with the issue of Reagan's compassion or fairness for low-income groups. He concludes that the Democrats won a debate on that issue. They succeeded in tagging Reagan as "unfair" to some groups.

Barrett finds that the opposition "emphasis on maintaining Social Security benefits intact was a bipartisan disservice to the commonwealth because it made all the more difficult the task that would soon have to be done, the curbing of future Social Security benefit increases among other changes in a system that needed overhaul."

Yet, Barrett admits that "in small, but measurable ways life was made more difficult for the very poor and somewhat easier for the wealthy" by Reaganomics. He acknowledges that "taxation trends and government spending demanded stringent examination," but he faults measures that fell heavily on programs involving the means test. The administration's handling of racial issues is faulted at many points for insensitivity and awkwardness, in spite of Reagan's personal soundness on the basic principle.

A chapter entitled "The Thing That Happened to Ronnie" is a great piece of narration that tells the dramatic story of the assassination attempt with skill and with some contribution to facts concealed or confused in the tumult of the time.

The budget battles with Congress are recounted with great skill and Reagan's budget compromises after the 1982 elections are described as a "mild mid-course correction under duress" that should have come six months earlier.

Passages in this book on the Reagan family life disclose a lot about the price that public men and women pay for their careers. In a democracy it is no doubt impossible to afford office holders at any level a great degree of privacy in their personal affairs. Politics, obviously, has cost the Reagans a great deal. One winces at disclosures of family affection and at hints of family discord, both of which one must wish might be dealt with more reticently. But these comments are the stuff that an inquisitive public most likes to read and maybe not too high a price to pay.

The organization of the book on which Barrett has lavished so much industrious inquiry and writing skill has a couple of confusing qualities. It bounces in and out of chronological sequence like a flashback movie scenario, and this gets in the way of a readily understandable narrative once in a while. There is, besides, a curious sort of fluctuating intensity in its registration. It challenges the reader to make quick shifts of perspective from a 35mm to a 200mm lens. One has the sensation of a man running through an art museum and giving passing pictures a glance while occasionally stopping to examine a masterpiece with a microscope. The fluctuating intensity and the shifts in sequence are bothersome. For all that, the book is a triumph of recollection and inquiry that will make it a rich source for all who write the second and subsequent drafts of a history of this period.