The book is a mixture of biography (he gives snapshots from his own life to support his thinking) and political manifesto in which he explains his ideas about education, health insurance, religion and the like.
I live in America, and reading this book would have helped me understand Obama as he ran for the Presidency. Now, however, it feels slightly out of date. The economic tsunami of 2007-09 isn't mentioned (the book was before it), and that impacts how you're going to read this book. You'll sit there thinking "have you really thought how much this is going to cost?"
You might argue that this book might help you understand his Presidency, and you're right I guess, but I'm in two minds about that. I have a nagging sense that this book might have been overtaken by events.