If you want to know more about positive psychology, the best overviews of the field are:

• Seligman, M. E. P. (2002). Authentic Happiness. New York: Free Press.
• A new, up to date textbook: Peterson, C. (2006): A Primer in Positive Psychology

If you want to know more about the topics covered in each chapter of The Happiness Hypothesis, I highly recommend these books:

Chapter 1. The Divided Self

• Damasio, A. (1994). Descartes' error: Emotion, reason, and the human brain. New York: Putnam.
• Gladwell, M. (2005). Blink: The power of thinking without thinking. New York: Little, Brown.
• Wegner, D. White Bears.

Chapter 2. Changing Your Mind

• Burns, D. D. (1999). Feeling Good (2nd Ed.). New York: Avon.
• Kramer, P. D. (1993). Listening to prozac. New York: Viking.

Chapter 3. Reciprocity With a Vengeance

• Carnegie, D. How to win friends and influence people.
• Cialdini, R. B. (2001). Influence: Science and practice (4th Ed.). Boston: Allyn and Bacon.
• Ridley, M. (1996). The origins of virtue. Harmondsworth, England: Penguin.

Chapter 4. The Faults of Others

• Baumeister, R. F. (1997). Evil: Inside human cruelty and violence. New York: W. H. Freeman.
• Wright, R. (1994). The moral animal: Pantheon.

Chapter 5. The Pursuit of Happiness

• Brim, G. (1992). Ambition. New York: Basic Books.
• Csikszentmihalyi, M. (1997). Finding flow. New York: Basic.
• Frank, R. H. (1999). Luxury fever: Why money fails to satisfy in an era of excess. New York: Free Press.
• Kasser, T. (2002). The high price of materialism. Cambridge, MA: MIT Press.
• Seligman, M. E. P. (2002). Authentic happiness. New York: Free Press.

Chapter 6. Love and Attachments

• Blum, D. (2002). Love at Goon Park. Cambridge MA: Perseus.
• Gottman, J. (1994). Why marriages succeed or fail. New York: Simon & Schuster.

Chapter 7. The Uses of Adversity

• Emmons, R. A. (1999). The psychology of ultimate concerns: Motivation and spirituality in personality. New York: Guilford.
• Frankl, V. E. (1984). Man's search for meaning. New  York: Pocket Books.
• Pennebaker, J. (1997). Opening up: The healing power of expressing emotions (revised edition). New York: Guilford.
• See this video, The Best Gift I Ever Survived

Chapter 8. The Felicity of Virtue

• Damon, W. (1997). The youth charter: How communities can work together to raise standards for all our children. New York: Free Press.
• Franklin, B. (1962). Autobiography of Benjamin Franklin. New York: MacMillan.
• Hunter, J. D. (2000). The death of character: Moral education in an age without good and evil. New York: Basic.
• Peterson, C., & Seligman, M. E. P. (2004). Character strengths and virtues: A handbook and classification. Washington DC: American Psychological Association, and Oxford University Press.
• Singer, P. (1979). Practical ethics. Cambridge: Cambridge University Press.

Chapter 9. Divinity With or Without God

• Abbott, E. A. (1952/1884). Flatland: A romance of many dimensions. 6th Edition. New York: Dover.
• Boyer, P. (2001). Religion explained: The evolutionary origins of religious thought. New York: Basic.
• Eliade, M. (1957/1959). The sacred and the profane: The nature of religion. W. R. Task (trans.). San Diego: Harcourt Brace.
• James, W. (1961). The varieties of religious experience. New York: Macmillan.
• Maslow, A. H. (1964). Religions, values, and peak-experiences. Columbus OH: Ohio State University Press.
• Miller, W. R., & C'de Baca, j. (2001). Quantum Change. New York: Guilford.

Chapter 10. Happiness Comes From Between

• Gardner, H., Csikszentmihalyi, m., & Damon, W. (2001). Good work: When excellence and ethics meet. New York: Basic.
• Leakey, R. (1994). The origin of humankind. New York: Basic Books.
• Richerson, P. J., & Boyd, R. (2005). Not by genes alone: How culture transformed human evolution. Chicago: University of Chicago Press.
• Wilson, D. S. (2002). Darwin's  cathedral: Evolution, religion, and the nature of society. Chicago: University of Chicago Press.