I can't quite say this book was life changing for me--because the first time I read it I was already starting to generally eat more whole, local food--but it did give me a manifesto: Eat food. Not too much. Mostly plants.
There's a bit more to it than that, but not much. Mainly it's about knowing the difference between "food" and "food-like substances".
To give you an idea, probably my very favorite of his "tells" is "Don't eat anything that won't eventually rot." It just seems like good sense, ya know?
Sweet Husband and I have gotten to where we even make a game of it sometimes. As in,
"Those Fruit Loops are so NOT food."
"Oh yeah, well neither are your Mike'n'Ikes!"
But all kidding aside, the book makes a very good case for eating real food, and it does so in a way that's not more judgmental than it has to be. For example, although Mr. Pollan frowns on high fructose corn syrup and the like, he's very realistic about it. I get the idea that even he has a handful of Mike'n'Ikes every now and then--just not every day.
In short, if I could recommend only one book to everyone from atop my "food issues" soapbox, this would unquestionably be it. You can see the rest of Mr. Pollan's "food rules" summarized here, but, really, just go read the book.