James Fallows is a national correspondent for The Atlantic and the author, most recently, of “China Airborne.”

During his 2008 campaign, Barack Obama so often stressed the improbability of his story that we have grown inured to how unlikely it really is. Everyone knows that his name, along with his inexperience, was an electoral handicap; that his mixed-race background made his victory historic; and that his transformation within five years from local Illinois politician to the most famous person on earth (and first incumbent president since Woodrow Wilson to win the Nobel Peace Prize) has no obvious parallel. The great virtue of David Maraniss’s huge and absorbing new biography is to demonstrate that Obama’s saga in its full and previously unexplored detail is more surprising and gripping than the version the world is familiar with.

The engrossing parts of “Barack Obama: The Story” are not the ones that created the most pre-publication buzz: the diary entries from one of Obama’s girlfriends in his New York days in the early 1980s. This was Genevieve Cook, a white Australian, who let Maraniss quote the notes she made during her infatuation with Obama and eventual estrangement — including the judgment that for all his initial charm, he proved to be too cool and distant. In the context of current politics, that may seem a relevant insight. But in the context of this book, those entries are almost ho-hum, precisely because they could have come from any troubled “it’s not about you . . . ” relationship. The rest of Maraniss’s chronicle, which very minutely traces the president’s African and American lineages back for more than a century, is far more unusual....