There is a lot of information out there (and by "out there" I mean "online") about diet – why or why not to eat meat, what it means to kill things, what we need or don't need in terms of a balanced meal, etc. – and it can be overwhelming.
So far, in my research, I've discovered five books that have been balanced, credible and not overly emotional or sentimental in their look at what we eat and why. I must admit these all basically advocate for eating less or even no animal products, but I think they do so in a way that's worth giving serious consideration.
1. Quantum Wellness, by Kathy Freston
2. Eating Animals, by Jonathan Safran Foer
3. The China Study, by T. Colin Campbell & Thomas M. Campbell
4. The Omnivore's Dilemma, by Michael Pollan
5. Spontaneous Healing, by Dr. Andrew Weil
What I think is particularly interesting about The China Study is that while it promotes the idea of eating less or even no animal protein, it does not advocate for a vegan diet. This might sound contradictory, but the whole point of the study was to document animal protein and its effect on human health – it doesn't touch the topic of animal welfare. And so, from a physician and researcher's perspective, all it's saying is we should be eating whole, plant-based foods. That's it. Deduce from that what you will, but here is a short, illuminating New York Times interview with the author, T. Colin Campbell, who explains this perspective.
Of course, PETA has its own recommendations (some of which overlap with mine) based solely on the animal cruelty argument for going vegetarian: http://prime.peta.org/2009/07/the-essential-vegetarian-reading-list