Another important paper in the field:
Axelrod R. & Hamilton, W. D. 1981: The evolution of cooperation. — Science 211: 1390–1396.
Unfortunately, I can't find a pdf of it.
Finally, here's an excerpt from John Maynard Smith's book
Evolution and the Theory of Games in which he discusses the evolution of altruism:
http://www.heretical.com/ess/evolcoop.html