The Hunger Games

The Independent attributes the success of The Hunger Games to sociology:

The real reason the books sell is because they deal with outsiders who buck convention. Teenagers want to break the rules, and that's not necessarily a criminal inclination any more. The Hunger Games highlights the angst, cynicism and grit of what being a teenager is really like. And it points to the fact that adolescent solipsism, bullying and a universal lack of empathy have also become signatures among grown-ups now, too.

Actually, people buy the book because it's a cracking good read.