Axelrod, The Evolution of Cooperation, (Basic Books, 1984). The best book available on the dilemma. Axerod is the political scientist who in 1980 ran a computer tournament in which programs played the p.d. against one another. The book analyzes the insights the p.d. gives us into society, nuclear strategy, biology and other fields.
Dawkins, The Selfish Gene(Oxford University press 1989). In this latest edition updating the seminal 1976 work, a chapter has been added discussing the implications of the p.d. for evolutionary biology.
Poundstone, The Prisoner's Dilemma(Anchor Books, 1993). This rather disorganized work of journalism does not know whether it wants to be a work of game theory or a biography of Von Neumann, a physicist and father of game theory who actually had little to do with the prisoner's dilemma itself. Nevertheless, it usefully describes the game, some variations and related concepts.
Wright, The Moral Animal, (Pantheon, 1994). This popular work on the evolutionary origins of morality relies on the p.d. as one illustration of the way ethics develops from biological origins.
You can play the prisoner's dilemma online against a computer opponent.
Principia Cybernetica has a listing on the p.d. which serves as a useful introduction to the concept.
A history of game theory contains a timeline with an extensive bibliography.
Lloyd Allison's Associative Memory is a searchable bibliography of scientific articles that contains numerous listings on the p.d.
A number of scholarly papers on the p.d. are avilable online. These include:
Sandholm, T. and Crites, R. 1995. Multiagent Reinforcement Learning in the Iterated Prisoner's Dilemma. Biosystems:Special Issue on the Prisoner's Dilemma.
Hal Varian and James Andreoni Pre-Play Contracting in the Prisoners' Dilemma
. Oliver Kirchkamp, Spatial Evolution of Automata in the Prisoners' Dilemma.
The Iterated Prisoner's Dilemma with Choice and Refusal page, run by Mark Smucker, contains links to many more papers available in Postscript format.