IMAGINE THAT the giants of industry - General Motors, IBM, AT&T - were privately held, family-owned corporations, not traded on any stock exchange, with no duty to report to stockholders and with very little accountability to the government. How much would we know about their operations? How would we assess their impact on the national and world scenes?

That fanciful situation is, in fact, a reality in the grain industry, and not just with a single, large company. All five of the firms that control the multi-billion-dollar world grain trade are family dynasties, operating under a shroud of secrecy, bearing no allegiance to any nation nor to any ideals other than their own considerable profits. Yet grain, writes Dan Morgan, "is the only resource in the world that is even more central to modern civilization than oil."

By their actions, the big grain firms can and do influence the economy and the domestic and foreign affairs of the United States, the world's largest grain exporter. They also determine the destinies of governments and the diets of people around the globe.

Even so, Morgan notes, most Americans have never even heard of the five grain giants - Cargill, Inc., of Minneapolis; Continental Grain Co., of New York; Andre, of Lausanne; Louis Dreyfus Co., of Paris; and Bunge Corporation, of New York and Buenos Aires.

In this extensively researched and highly readable book, Morgan, a Washington Post reporter, strips away some of the grain industry's anonymity with a revealing examination of the handful of powerful people who control the world's wheat, corn, soybeans and rice. Merchants of Grain adds a valuable chapter to the slowly growing literature on today's great multinational businesses.

In addition to his lengthy, historical portrayal of the grain trade and the families who direct it, Morgan has set down the details of some major contemporary events - the Soviet "grain robbery" of 1972. the U.S. grain export scandal that rocked the big firms in 1975, the Tongsun Park rice shipping and influence peddling case, and some fascinating material on one of Henry Kissinger's less glorious hours.

This last involves the former secretary of state's attempt, in 1975, to refuse American grain to the Soviet Union, unless the Russians would agree to sell the U.S. ten million tons of oil a year at a substantial discount from the world market price. In Morgan's view, the attempt was foolish from both an economic and political standpoint. Treasury Secretary William Simon, writes Morgan, believed that Kissinger's effort amounted to "extortion," and "thought the diplomatic genius at the State Department had gone slightly mad."

Long after it became clear to nearly everyone else that the Soviets would tighten their belts and get by on less grain rather than yield on oil prices, Kissinger still refused to let his negotiators come home from Moscow.

Finally, President Gerald Ford woke up to the growing frustation in the U.S. farm belt over the embargo on grain sales to the Soviets. He called off Kissinger, gave full control of both domestic and international agricultural policy back to Agriculture Secretary Earl Butz, and allowed the grain firms to resume sales.

Morgan speculates, however, that the whole incident may have cost Ford the presidency. In a desperate attempt to shore up wavering Republican support in farm states, Ford chose Senator Robert Dole of Kansas as his vice-presidential nominee, a selection that "precluded a running mate with appeal to voters in the big East Coast states that the President lost."

The massive 1972 U.S. grain sales to the Soviet Union were engineered under cover by the big grain companies. But the author concludes that top officials in Butz's Agriculture Department must have known in advance, but "covered up," the extent of the Russian raid that reduced American grain stocks to dangerously low levels.

No "smoking gun" proving criminal collusion between the grain firms and the U.S. government has been found, but "it is clear from the record that there was bureaucratic negligence and the appearance of conflicts of interest," Morgan writes. He documents thoroughly the ties between the Agriculture Department hierarchy and Continental Grain, the firm that gained the most in the sales. Several of the key Butz aides of those days, incidentally, returned to work for the grain companies which had profited so handsomely - an example of the constantly revolving door between the Agriculture Department and the firms.

Only two weaknesses slightly mar Morgan's admirable book. The pervasive influence of the big grain firms and their trade associations on domestic agricultural policy, including farm legislation and grain-trade regulation, is largely overlooked. The book focuses instead on the companies' role in the international scene - reflecting Morgan's own interests and The Washington Post's coverage.

Also, the book paints an unjustifiably sympathetic portrait of Edward (Ned) Cook of Memphis and his firm, Cook Industries, Inc., which tries to shift from the cotton business to the global grain trade and got stung in the process. By this account, Cook Industries was a struggling, upstart company, shut out of the grain business by the powerful and evil Big Five. In fact, Cook Industries was at least as corrupt as several of the bigger firms.

In 1976, Cook Industries was convicted and paid a $370,000 fine for having systematically cheated its foreign customers by "shortweighing" grain ships bound for overseas from its New Orleans elevator. Morgan notes the conviction, but he implies that Cook Industries was drawn into the corruption against its will and argues the government claims against the firm were "inflated." Most of the other export grains firms were also convicted in what became one of the biggest agriculture-related scandals in U.S. history. It brought the secretive grain companies into public view in a way that darkened public images already tarnished by the "grain robbery" of a few years earlier.

The affair illustrated dramatically the lengths to which the companies would go to set up an inspection system that they themselves owned and controlled, and then still resort to methodical bribes, misweighing and adulteration of grain. Poor countries, for whom food aid was critical and who could least afford to complain, were cheat the most - receiving rotten grain and shortweighed shipments from U.S. ports.

The activities were so blatant that Congress responded with a new federal grain inspection system. In this case, the grain giants went too far and got caught. But for the most part the merchants of grain still operate, aroung the world, largely free from control by the U.S. government or that of any other nation. They are likely to continue doing so. But with Morgan's fine book in print, we at least have the opportunity of knowing something of how they do it. CAPTION: Illustration, no caption