The President has given me permission to take a kind of voyage with him--to watch him closely through a working week....I will be with him, most of the time, hour in and hour out....   At 8:33 on a rainy Monday in March, 1975, John Hersey sits down on a straight cane-backed chair in the Oval Office to begin soaking up impressions of what happens--in post-Watergate Washington--at the center of American power. Through five and a half days, he will stay close to the President, observing him as he consults with his own staff, with members of Congress, with his Cabinet, with Rockefeller; watching him on the exercise bike, at the barber's, greeting Miss America: absorbing his confidences as he talks after dinner, in the private quarters of the White House, about his childhood and about his college years when it was difficult to make ends meet.              Following the President, Hersey observes in detail all the important moments--as well as the incidental ones--that show what Gerald Ford is like on the job. In this extraordinary book he builds a brilliant and revealing portrait, letting the reader see Ford's strengths and limitations. And so perceptively does Hersey draw significance from his observations that the insights seem to explode like time bombs.    I have seen all week that it is not easy for Gerald Ford...to make what he refers to, in the language of umpires, as "a tough call." Yet once he has made such a decision, he does not agonize...he becomes convinced of its rightness and is stubborn in its defense....   In reading The President, each of us emerges knowing more than ever before, not only about this imperturbable "iron" man, the first President we did not elect, but also about how the Presidency really operates. In John Hersey's report we come to understand--the man, and the things that persuade him. And we come to sense...how good it would be if in some way he could speak--good listener that he is--one-to-one with ordinary men and women, his constituents, from whom he has somehow drifted so far away.