ROOSEVELT TO REAGAN; A Reporter's Encounters With Nine Presidents By Hedley Donovan. Bessie. Harper & Row. 333 pp. $19.95.

I N THIS SERIES of essays, Hedley Donovan, successor to Time Inc. founder Henry Luce as the magazine's editor-in-chief, offers entertaining and sometimes slightly pompous insights into presidents he has known. For White House junkies, which must include almost everyone inside the Beltway, the book is recommended summer reading.

Donovan starts with Franklin Roosevelt and ends with Ronald Reagan. If there is a theme, it deals with what the author calls "the abiding paradox of the U.S. Presidency." Although "the most powerful political office in the world," it is "hedged about by a mighty host of contending powers: Not just Congress, but the bureaucracy, the press, business, the courts, lobbies, the great American electorate itself, and then all the other countries on earth."

Nothing new in that. Likewise, until he gets to Jimmy Carter, Donovan's opinions and personal presidential experiences, if not precisely run of the mill, are not breathtaking, either. A few samples:

In a genial admission of error, Donovan says that what he allowed to be printed in the February, 1952, Fortune about Harry Truman now looks "downright embarrassing," i.e., "The precise shading of mediocrity to be assigned to him may well occupy the historians for many years." Today he ranks HST in the "Good to Very Good category of Presidents."

Dwight Eisenhower, whom Time Inc. would have invented had he not come along, was "a brilliant politician" who liked to think himself above politics. But Ike, unlike Truman, never "rose to his full potential."

Kennedy admirers will not like the Donovan verdict. He strongly doubts that Kennedy could ever have been a great president; there were fewer accomplishments in his three years than in the presidencies of Lyndon Johnson or Richard Nixon, one of whom left the office "broken," the other "in disgrace." Kennedy's "greatest achievement," decrees Donovan, "is precisely his legend . . . the romantic hero, martyred and forever young." Kennedy on taxes or on civil rights? On defense and the Soviets? The accomplishments are smothered by Donovan's disgust with what he calls "flaws in the Kennedy character" and White House secret tapings. Donovan reports the August, 1983, Harris poll -- showing that JFK led all the "Roosevelt to Reagan" presidents as the one who "most inspired confidence in the White House" with 40 per cent, followed by 23 percent for FDR -- as "an astounding judgment."

Hurrying along the Donovan presidential trail: LBJ -- this "in some ways monstrous man" capable of "deeply generous impulses and great visions for America" -- might have provided Shakespeare with "the richest theater" of all.

Donovan thinks Gerald Ford "overpraised" for his "healing role" after the long dark night of Watergate. "Unless Nixon had been succeeded by, say, Bob Haldeman or John Mitchell, no successor could have failed to 'heal'."

Momentarily skipping Jimmy Carter, about whom Donovan has interesting things to say because he worked for him in the White House, we come briefly to Ronald Reagan -- "unmistakably a leader." His final place in history may well depend on how much of him, "phenomenon that he is, is politically or philosophically transferable" to the post- Reagan era.

DONOVAN on Carter is the rich dessert that awaits the unsuspecting reader of Roosevelt to Reagan. In 1979, at the height of the Carter-induced crisis of "malaise," the president recruited Donovan, just retired as editor of Time Inc., to become his senior adviser, reporting only to Carter himself. Listing plusses and minuses of taking the job, Donovan wrote a last minus: "Titanic?"

"I wasn't really much concerned with the then low state of the President's political fortunes," Donovan writes. "I would not have wanted to stay into a second term even if he were reelected, which seemed very unlikely in midsummer 1979." Why, then, did he accept the job? Because he would kick himself "around the block the rest of my life if I didn't give it a try."

So courtly Hedley Donovan went to work that August for the populist, born-again Democratic president. Too bad, in retrospect, that Carter failed to take more of the solid advice he got in some profusion from his senior adviser. Donovan found it "deplorably naive" that Carter, surprised by the Soviet invasion of Afghanistan, told an ABC interviewer that Afghanistan "had changed his whole thinking about the Russians" and that the nature of Soviet imperialism was "only now dawning on the World."

Donovan was appalled that Secretary of State Edmund Muskie believed the Soviet invasion of Afghanistan made ratification of SALT II even more important. Donovan dashed off a note to Carter: "Would another Afghanistan somewhere else make SALT ratification still more important? How far would we carry this counter-linkage?"

When Donovan told Carter in February, 1980, that the crises in Iran and Afghanistan had trapped presidential advisers in day-to- day tactical politics, ending all systematic thinking about long-range foreign policy, the president directed him to do some long- range geopolitical planning. The top-secret report Donovan submitted just before the end of his one-year stint with Carter was written without fear of pain or punishment, proving that a lucky president can get unbiased recommendations from outside the chain of command.

The editor struck at the very policy foundations of the president. He argued that the U.S. must be "the preeminent superpower"; that the Soviet Union was "rapidly achieving, if it has not already done so, superiority over the U.S."; that "in some of our (human rights) rhetoric, the brute totalitarianism of the Soviets seems to get equated with a traditional Latin America military junta because they are both offenders against human rights."

No shading of the truth there. In Carter's last acts before the curtain descended on his doleful four years, at least some of Donovan's courageous advice was heeded. Salt II was withdrawn from the Senate, and defense spending was radically increased. Shortly after submitting his report, Donovan left the White House, not wanting to be caught up in the 1980 campaign. He voted for Carter with minimal enthusiasm and summed him up: "How could a man be so very bright and still fail to get a good grip on the fundamentals of his job?"

That verdict on the only president Donovan could claim to know well seems to fit his generally low regard for all presidents since Eisenhower, or at least for their failure successfully to "fight the idea that the Presidency is unmanageable." The author drives the dark thought deeper with this unhappy but accurate conclusion: "No American under forty has adult experience of a broadly successful presidency."