Geoffrey Miller's The Mating Mind impressed me. It was clearly intended for a general audience, and so light on the really detailed evidence, but Miller showed an awareness not shared by the other evolutionary psychologists I've looked at to recognize when his data wasn't all that and be cautious with his speculations. His speculations were nonetheless fascinating and in many cases plausible.
In particular, I am attracted to his theory that humans might have evolved ethical behavior as a mating display. Altruism obviously involves taking on costs for no apparent benefit, and it is less often noted that the same is true of punishment in most instances. One of the depressing aspects of this theory, but also one of my reasons for finding it extremely plausible, is that it accounts for why people generally devote so little effort to making sure their generosity and punishments are directed at the most appropriate targets. If we punish people to show off our surplus power, and perhaps our commitment to the community, we're going to want to punish unpopular people (harming popular people will have negative effects which would undermine the benefits of the display), but punishing people who are actually guilty, or whose behavior is likely to be usefully influenced by the punishment, would hardly matter at all. That is a pretty good description of how people target their punishments.
Of course, I also take this as further evidence for utilitarianism. The objections which are always considered so problematic for utilitarianism involve cases where utilitarianism gives answers that feel wrong. If Miller's right about where our ethical feelings come from, my own inclination to think people shouldn't be putting so much faith in their unanalyzed feelings seems even more justified.