Whenever America is polarized, as it is today, people go back in memory and history to recall other times their nation was so divided. The Civil War of the 1860s and the social revolution that tore us apart in the 1960s come instantly to mind. In that latter time, there was no figure more central to the conflicts of his day than Richard M. Nixon. And no staff member was closer to Nixon in the campaign of 1968, or for the first four years of his presidency, than his personal aide Dwight Chapin, whose memoir, "The President's Man," is published...