In the early 1970s the international relationships that had been frozen since soon after World War II showed signs of decongealing. Sensing the historical moment, the Nixon administration aspired to be both the catalyst and the beneficiary of this time of rare opportunity. As national security assistant and then secretary of stale, Henry Kissinger was uniquely situated to participate in, shape, and observe these efforts. The first volume of his memoirs, White House Years, detailed his part in the early Nixon period, January. 1969, to January, 1973. Now he continues the story in a weighty second volume. Years of Upheaval (Little. Brown; 1,283 pp.; $24.95) takes us from 1973 to the Nixon resignation in August, 1974.