Alfred Jules Ayer, Language, Truth and Logic (New York: Dover, 1952). Ayer argues that only semantic propositions can be proven, while the best we can do about empirical matters (even gravity) is to entertain theories. Moral statements he calls mere meaningless statements of opinion.
Richard Dawkins, The Selfish Gene (New York: Oxford University Press, 1989). An excllent summary of evolutionary theory, with an attempt to account for the existence of altruism.
T.H. Huxley, Evolution and Ethics (Princeton: Princeton University Press, 1989). Huxley's 1892 beautifully written argument for the inconsistency of ethics and evolution; ethics is what we invent to get out of the evolutionary conflict, he says.
Peter Singer, Practical Ethics (New York: Cambridge University Press, 1993). The controversial Singer is always thoughtful and very lucid on matters of modern morality, though I disagree with many of his conclusions.
Peter Singer, The Expanding Circle (New York: Farrar, Straus and Giroux, 1981). Unfortunately out of print, this book posits that our altruism, originally limited to our closest kin for reasons of evolutionary survival, has expanded to include all living things.