Some interesting background on Obama the author from The New York Times. The article includes this:
The books have defined Mr. Obama’s public image in a way that few books by politicians have done. Reporters paw through them for insights into Mr. Obama the candidate, supplied by Mr. Obama the author. Out of his story, he has also drawn the central promise of his campaign: if a biracial son of a Kenyan and a Kansan could reconcile the seemingly irreconcilable in himself, a divided country could do the same.
His memoir is, as one publisher put it, “the single most vetted book in American politics right now.” Written at a time when Mr. Obama says he was thinking less about a career in politics than about simply writing a good book, it leaves an impression of candidness and authenticity that gives it much of its power.