Spectacular international crises and the nuts-and-bolts diplomacy that averted some of them are both chronicled in this restrained memoir of the Burmese dignitary's 1961-71 term as UN Secretary General. Most striking are the accounts of U Thant's 1964 efforts to arrange secret talks between Hanoi and Washington (which killed the idea without President Johnson's knowledge, he says) and of personal success during the 1962 Cuban missile showdown in convincing Castro to moderate his public complaints about Khrushchev. In his detailed treatment of the Middle East, Thant rather defensively but quite convincingly insists that he had no choice about pulling UN troops out of Sinai and Gaza in 1967, and, in summation, balances the fate of the Palestinian refugees with that of Soviet Jewry. Other cases where the organization was prevented from attempting to keep or restore the peace are reviewed, including the Czechoslovak events of 1968 and the Dominican revolt of 1965; Thant believes that, along with global economic development. ""preventive diplomacy"" of a kind requiring courageous statesmen is the key to the UN's potential. His judgment of individuals is sometimes tart; Dean Acheson was ""one of the most overrated diplomats of his time"" and LBJ was ""juvenile in his concept of international developments."" while Adlai Stevenson gets high marks and Premier Kosygin is unmatched in ""quiet dignity and courteous manner."" Yet though clearly a shrewd mediator, Thant himself displays above all a sometimes annoying, sometimes engaging quality of artlessness--""Imagine the King of Jordan addressing a former school-teacher with a profusion of 'sirs!',"" he writes, and revels in the shock lie precipitated by publicly calling Moishe Tshombe's Katanga secessionists ""a bunch of clowns."" A valuable cross-check source for specialists, a thoughtful retrospective for generalists.