I'm not sure what to make of this book.

It certainly tells you what you want to hear - dieting is pointless, you probably weight about what you're going to weigh regardless, no drastic changes necessary. And she seems to back that up with plenty of facts and studies.

On the other hand, she claims it's really hard to gain weight and has the studies to back that up. The obesity rates in the US would seem to disagree with that. So do you believe her or not?

Probably yes. It's better to adopt good habits, employ strategies to avoid bad habits, and exercise. Do that and your weight is likely to take care of itself.