When President Truman in the spring of 1951 was weighing the decision whether to fire Gen. Douglas MacArthur from his command of United Nations forces in Korea, he sat down one Sunday afternoon with Speaker of the House Sam Rayburn and Chief Justice Fred Vinson. Both urged him to be cautious because of the domestic political dangers.

But Harry Truman had already made up his mind. As he later explained in a letter to Dwight D. Eisenhower, "I was sorry to have to reach a parting of the way with the big man in Asia, but he asked for it, and I had to give it to him."

Thirty years later, in the autumn of 1981, President Reagan met in the White House living quarters with Secretary of Defense Caspar L. Weinberger to make a decision on the multi- billion-dollar MX missile plan, a key component of his national security strategy. Weinberger, sensing the president's weariness, did not spend much time on the merits of the alternative deployment schemes the Pentagon and Congress had been debating for years.

"Instead, Weinberger hauled out a cartoon drawn by Mike Keefe of the Denver Post which showed Uncle Sam playing a shell game with a Russian, inviting him to guess which shell concealed the MX missile. The Russian in the cartoon takes out his hammer and destroys all the shells. Reagan chuckled, and approved the Weinberger plan."

The Truman anecdote comes from "Tumultuous Years," the concluding second volume of Robert J. Donovan's biography of the man from Missouri. The description of Reagan's MX decision-making is quoted from Lou Cannon's biography, "Reagan." Both books have just been published, and if a prejudiced party may say so, they are models of craftsmanship and insight -- telling more about the workings of two presidential politicians than anything you are likely to have read in years.

I am a prejudiced party, because both books were written by journalistic colleagues and friends. Before "retiring," Bob Donovan was the Washington bureau chief for the old New York Herald Tribune and the Los Angeles Times. Lou Cannon is the White House correspondent for The Post.

Donovan and Cannon have managed to cross the bridge from daily newspapering to history and biography without losing their balance, and that is a rare enough kind of event to be celebrated by all of us in the business.

From the reader's standpoint, the great attraction of these books is the intimacy of the view they give of politicians and presidents at work.

These are not debunking books. Donovan and Cannon do not have a cynical bone in their bodies. But they are not filled with romance or reverence either. They are perfectly clear-eyed and persuasive portraits of two very different individuals dealing with the pressures and perils of an extremely tough job.

Donovan has been rummaging in the Truman Library in Independence and reading through hitherto closed papers of former press secretary Roger Tubby and former Truman aides Stuart Symington and W. Averell Harriman. Cannon has exploited his own reporting notes from Reagan's California and national campaigns and his tenure in Sacramento and Washington. He has talked with every insider at the White House and almost every living person who played a significant part in shaping Reagan's show business and political careers.

You come away from reading these books with the feeling that you understand these two presidents in a way you never did before. And when you have finished, you have been persuaded that these are two presidents who have mattered.

Donovan concludes his book this way: "Probably more than he realized, Truman cast a long shadow across the history of the second half of the twentieth century. That history is not always written in a way that Mr. Truman would have approved. In the tormented epoch that began in 1914, no President has emerged from the office as a saint or a hero. Surely, for many reasons good and bad, for his serious mistakes as well as for his wise decisions, not to mention his vintage American character, Truman's was a most extraordinary presidency."

Writing about a different kind of "American character," Cannon says, "I believe that Reagan will not run again, but I also think that the impact of his presidency will resonate for many years. Despite his failings, Reagan was a presence in the American political system from the moment he made his historic speech for (Barry) Goldwater on Oct. 27, 1964. He had a vision of what America had been and what it should be again, and he tried to translate this vision into reality. The government of California was not the same after Reagan left it, and the government of the United States will not be either. . . . Reagan has made a difference."

These books make a difference in our picture of two presidents. By going well beyond journalism, these two journalists have rendered a real service.