AT A CROWDED Washington press conference last month, a dozen members and advisers of a prestigious presidential commission reported their unanimous recommendations on what kind of new missiles the United States should rely on for its future security.

Among them, was Harold Brown, the nuclear physicist and former secretary of defense in the Carter administration.

Midway through the conference, aides handed reporters a separate statement by Brown in which he reaffirmed his support for the commission's work but added warnings that went well beyond those in the official report.

Issuing that separate, more cautious statement was vintage Harold Brown, probably what has now become an involuntary reflex by a scientist-manager who is so bright that he sees every side of every issue to extraordinary depth. It is a trait that makes him always awesome intellectually yet frequently frustrating to admirers and critics alike.

Zbigniew Brzezinski, who served as Carter's national security adviser and was a colleague of Brown's, probably best summed up this brilliant yet enigmatic figure in his recent book about the Carter era, Power and Principle.

"The President's frustration with Harold's ambiguity was in part justified," Brzezinski wrote. "I also felt that out of personal caution and intellectual agility Harold tended to see every side of an issue, edge up to a firm view, and then quickly again hedge it."

Now Brown has written his own book: Thinking About National Security, Defense and Foreign Policy in a Dangerous World and in some important ways it is in keeping with Brzezinski's earlier assessment.

Indeed, at the outset Brown warns us not to expect definitive answers to some of the most mind-boggling questions of this dangerous nuclear age. Characteristically, he says, there is "no generally agreed upon answer" to the question of how much is enough to continue deterring nuclear attack. And there are "no precise criteria" to define the proper overall size of the American nuclear strike force or the balance within it of the land, sea and air elements.

His purpose in writing "is to help establish a framework for thinking about national security. Public and political leaders need such a framework more than they need a prescription for what to think."

Perhaps. But Brown--a wunderkind PhD in physics at age 22, a director of the Livermore Radiation Laboratory for a dozen years, a man who has seen a dozen atomic bomb tests and says "you don't forget that easily," a former chief Pentagon scientist, secretary of the Air Force, president of the California Institute of Technology, and secretary of defense--is one of the very few people who has the credentials and the respect to write such a prescription.

The fact that he does not, is something of a disappointment.

Brown's book, however, is not a disappointment. It is a valuable book, a road map through the difficult choices and pressures that civilian Pentagon leaders face and the political, military, economic and foreign policy realities that limit their ability to cope.

Although Brown's "on the one hand, on the other hand" analysis of every problem is frequently maddening to the reader, it does convey well his point that there are few easy answers.

Unlike the trend in books by former officials, this one is free of personalities. It is all business. Brown's 288- page volume is also the first major reflection by a former defense secretary since Robert S. McNamara's The Essence of Security appeared in 1968.

Brown is not a lively writer. But he is eloquent in stressing that security can only rest on the nation's internal political and economic strength and that public consensus and rationality in defense are necessary for the kind of long-term popular support that produces results. Both the United States and its allies, he says, have seen too little leadership.

Brown argues that the search for economic fairness must be pressed for America's growing minorities if this country is to be able to retain domestic support needed for future defense and foreign aid requests.

He warns American allies in Western Europe and Japan that they must do more; that this country cannot be seen as more interested in their security than they are.

Brown provides fresh insights into the complicated military-foreign policy linkages with Japan, China, South Korea and the Persian Gulf but, disappointingly, fails to offer any insight into the U.S. military role or risks in Central America.

He is sharply critical of the traditional decision-making procedures of the military Joint Chiefs of Staff and the quality of judgments it produces. The chiefs, he says, face an "insuperable conflict of interest" by also running their individual services, and he adds his voice to those already calling for change.

To critics of high-technology weaponry, Brown argues that the United States has no choice but to exploit its technological edge to balance the numerically superior Soviet forces. But he urges against ruining new weapons by always seeking that extra 5-to-10 percent of performance that ultimately leads to battlefield breakdowns.

He supports 3-to-5 percent real growth as a minimum for annual defense spending increases that can be sustained even under poor economic conditions and 5-to-7 percent as a goal for better times. Both are less than Reagan administration objectives.

Brown is critical of administration plans to expand the number of Navy aircraft carrier task forces from 12 to 15, says its a mistake to take battleships out of mothballs, and is critical of officials "who assert U.S. inferiority or understate U.S. military capabilities" for a variety of reasons and thus "damage U.S. interests."

Brown, however, is no dove. Furthermore, he is a big believer in perceptions; that what the Soviet Union and other countries think about the U.S.-U.S.S.R. balance of power is perhaps as important as the real balance. So he continues to believe in what he calls "the paradox of the nuclear age: a need for massive expenditures to ensure that the resulting forces are unuseable by either side as instruments of policy."

In battlefield terms, running the Pentagon is almost certain to be a no-win situation, no matter who occupies the secretary's office. Former Nixon-era secretary Melvin R. Laird, perhaps the most politically skillful occupant ever to hold that office, used to muse privately, and correctly, that the secretary's job was no platform from which to seek higher office.

Given the virtual certainty of controversy, we basically have to be grateful for whatever small blessings any secretary can make toward improvements in defense policy and operations. Robert McNamara brought a management revolution. Laird and Clark Clifford brought political skills at important times. James R. Schlesinger and Harold Brown, probably the two officials who were most qualified in terms of experience to be named to that post, have brought a measure of thoughtfulness about the dilemmas of nuclear strategy.

Now Brown has delivered his small blessing, a volume that you can reach for on the shelf anytime, no matter what side of the issue you are on.