I would second Omnivore's Dilemma, excellent.
If scientific data and studies on why things are really great for you, I highly recommend the Perfect Health Diet. (The Jaminet's also blog extensively, so google it and take a gander at what they're about.)
Nourishing Traditions by Sally Fallon is a gold standard for food politics/reclaiming traditional knowledge of food preparation, though the book is mostly recipes.
Another site that always gets me super-psyched to eat well is World's Healthiest Foods (whfoods.com). It's basically an encyclopedia of why X fruit or vegetable is healthy for you! It also has a nifty calculator where it will ask about your current diet, estimate nutrients you might be deficient in, and provide healthy recipes for foods that are high in the nutrients you aren't eating a lot of. I LOVE that site.