Most intellectuals are attracted to power, but none with less ambivalence than Arthur Schlesinger Jr., the brilliant historian of the New Deal who became the quasi-official chronicler of the Kennedys. Schlesinger was born into an American aristocracy in its prime — the Eastern liberal intelligentsia. He was the son of an eminent Harvard historian and grew up apparently knowing everyone. He met his first Kennedy (sister Rosemary, before her lobotomy) when he was 14, at a Christmas party in 1931. It was natural, maybe even inevitable, that Schlesinger, while serving as an intelligence officer in Germany at the end of the war, would be invited to dinner “at the palatial mansion shared by George Ball and Ken Galbraith.” In Washington after the war, the young author of “The Age of Jackson” had a place at the table of Georgetown dinner parties thrown by the columnist Joseph Alsop, where he met Democratic Party royalty — Averell Harriman (“quite favorably impressed by him”), Franklin D. Roosevelt Jr. (“looks astonishingly like his father and has adopted many of his mannerisms”) and John F. Kennedy (“seemed very sincere and not unintelligent, but kind of on the conservative side”)....