In Defense of Food is one of those books that right away makes you pay attention and assess your life and diet based on its message.
The book's conceit and overt motto is elegantly simple: Eat food. Not too much. Mostly plants.
What this translates to is a call to replace much of the standard, additive rich fare in the Western Diet with real, traditional food, grown naturally and purchased locally if possible. In addition to converting to organic or natural food, the author suggests investing more time and resources into meal planning and eating, and consuming more vegetables (especially greens).
It's hard to conceive of a reason not to read this book, unless you're someone who's already digested the message or, perhaps, you have a vested interest in the continued dominion of agribusiness.