The Stanford Encyclopedia Articles on the course website are probably the best places to start looking for extra readings on the units of selection or on biological altruism. But here are a few of the many other important or interesting pieces. Please ask me if you are looking for something on a specific topic or if you want to get a sense of how things fit together.
Here are excerpts from George Williams book Adaptation and Natural Selection which argues, roughly, that group selection and group adaptations are very rare or perhaps don't exist at all. Richard Lewontin's piece is perhaps the first to try to lay out the abstract logic of natural selection in a way that favors thinking about multiple levels and he has a discussion of some interesting cases like the t-allele in mice. The first two chapters of Dawkins' The Selfish Gene introduces what he means by altruism and what he is going to argue for in the book. And he says what he means by replicator which is really the foundation of his view. Chapter 6 is where he introduces kin selection and inclusive fitness theory. The Sober and Lewontin piece uses heterozygote superiority as an example of something that the gene's eye view can't handle properly (it is this piece that Sterelny and Kitcher are attacking). This David Sloan Wilson piece lays out his trait-group model which is a way to think about group selection.
The original papers on inclusive fitness theory are from W.D. Hamilton (and Maynard Smith taking the idea from Hamilton):
The first paper to carefully discuss reciprocal altruism is from Robert Trivers:
Robert Axelrod and William Hamilton used reciprocal altruism ideas to model the evolution of tit-for-tat strategies in iterated prisoner's dilemmas:
The Axelrod and Hamilton research turned into a much longer and very readable book which includes several chapters on political science and public policy recommendations as well as on the prisoner's dilemma and evolution. He also wrote a 'sequal' which is a collection of his papers.
Elliott Sober and David Sloan Wilson's Unto Others: The Evolution and Psychology of Unselfish Behavior discusses both biological and psychological altruism and the relationship between the two.
Recently, E.O. Wilson has argued against his former self by joining David Sloan Wilson in thinking that group selection may be a better way of thinking about altruism in nature after all. The first piece below is the main work here, the other two are shorter, popular versions of the same basic ideal:
In addition to the above pieces, E. O. Wilson (and others) have a few other pieces arguing that, in particular, kin-selection can't be the explanation of eusociality:
These papers have been entertainingly criticized by Richard Dawkins and Jerry Coyne in their respective blogs and the last piece in particular by an avalanche of papers (from scientists) written to Nature. The link here will get you to the five published replies (the first has, by my rough count, 143 authors) and the response of Novak et al.
The group selection debate videos that I mentioned are here: