When acclaimed Washington Post writer Wil Haygood had an early hunch that Obama would win the 2008 election, he thought he'd highlight the singular moment by exploring the life of someone who had come of age when segregation was so widespread, so embedded in the culture, as to make the very thought of a black president inconceivable. He struck gold when he tracked down Eugene Allen, a butler who had served no fewer than eight presidents, from Harry...