A Life in the Twentieth Century: Innocent Beginnings, 1917-1950, Arthur M. Schlesinger, Jr. Houghton Mifflin, 557 pages, $28.95.

I first met Arthur Schlesinger, Jr., 54 years ago at the founding convention of Americans for Democratic Action (ADA) in Washington, D.C. I was dazzled by the array of notables in attendance--from Eleanor Roosevelt to the young mayor of Minneapolis, Hubert Humphrey--and even in that goodly company no one could doubt that Schlesinger, barely 29, was regarded as one of the elite.