One of my major interests is how diet affects health, and I have read a lot of books on this topic. Most recently, this book by Michael Pollan, who also wrote The Ominvore's Dilemma (which I haven't read). In Defense of Food is excellent. He doesn't try to scare people into veganism, but he shows very reasonably, how we can make better food choices, which affect not only our own health, but the whole food chain, in a positive way. His basic premise is: "Eat food. Not too much. Mostly plants."
"Eat food" refers to Pollan's opinion that "the most important fact about any food is not its nutrient content but its degree of processing". Therefore we should be eating food as close to its natural state as possible, and avoiding processed food "products".
"Not too much" refers to Pollan's thoughts on good eating habits: eating regular meals instead of grazing all day long, eating less food but better quality, sitting down at the table, eating slowly, cooking at home, and even gardening.
Finally, "mostly plants" means just that: "a diet rich in vegetables and fruits reduces the risk of dying from all the Western diseases." Pollan does not require full fledged vegetarianism or veganism - eating very small amounts of meat is still acceptable and beneficial in his book.
I found this to be an eloquent exploration of our modern eating habits and how we do have the power to make small changes that have enormous impact. Read more about my own personal take on food at my health and fitness blog here: http://fitandfabulousatforty.blogspot.com/2008/03/transition-from-flexitarian-to.html