“When I was a kid,” David Greenberg recalled in a recent e-mail, “my perceptions of Nixon were shaped as much by ‘Saturday Night Live’ and songs like ‘Ohio’ (‘Tin soldiers and Nixon coming. . . . ’) as by political journalism, which I didn’t read until I was older.” In the intervening years, Greenberg, whose book “Nixon’s Shadow: The History of an Image” was published in 2003, has had plenty of opportunity to consider the various ways we view the presidency...