The true answer to the question "What book changed your life?" is "Almost every book I've read." I think my personal definition of a good book is one that has changed my perspective somehow. So when I first saw this topic, dozens of books ran through my head. But I chose Fast Food Nation because it gave me a big push to change physical aspects of my life, not just mental.
I had already stopped eating beef when I read Fast Food Nation: The Dark Side of the All-American Meal by Eric Schlosser. My reasons were fuzzy and hard to explain to other people, but this book gave me sound-bite-worthy answers to their constant questions. If they thought that 'saving the environment' wasn't a good reason, I'd talk about the horrific working conditions in American slaughterhouses. If they thought that was stupid, I'd talk about food safety and how no one in this country is testing for Mad Cow, and therefore no one is finding it - and how E. coli is a direct result of the factory farming system. And on and on.
After reading Fast Food Nation, I quit eating at all fast food restaurants. What I learned in this book helped me to stay on course with my healthy food choices when I was tempted to eat junk. It was well-written and informative, so I started reading more non-fiction (especially food-related books, such as The Omnivore's Dilemma by Michael Pollan and Animal, Vegetable, Miracle by Barbara Kingsolver). I shop local and organic at every opportunity. The way I shop and eat has completely transformed in the last 10 years, and much of that can be credited to Fast Food Nation.