I'm currently reading The Ethics of What We Eat (published under a different title in the US) by Peter Singer and Jim Mason. I'll be reviewing this elsewhere, so I won't comment at length here.
The book brings home why I admire Singer so much - enormous effort must have gone into the research, and the whole thing is written in a way that is very understanding to people who don't meet the authors' view of what, morally speaking, might be the best food choices in all the circumstances. Ethical omnivores need not feel shrilly lectured to by the vegan authors, who are not going to demand total change in anybody's life, and less-ethical-than-we'd-like-to-be omnivores are allowed to ... well, simply wriggle in our seats, while also worrying about the cumulative picture building up to suggest we need to make some changes. And build up it does. This is advocacy of a very powerful kind, partly because of its relative gentleness.