The psychology of religion is now a respectable topic within psychology and courses in this area are taught at most divinity schools. There are many competent books on religious belief and the general psychology of religion.
Batson, C. Daniel, Schoenrade, Patrica, & Ventis, W. Larry. Religion and the Individual: A Social-Psychological Perspective. Oxford University Press, 1993. This is a second edition of their earlier The Religious Experience: A Social-Psychological Perspective. Oxford University Press, 1983. Both deal with a range of issues in the psychology of religion such as sources of religious belief, social supports for religion, and the effects of religious belief on behavior. While this book is a bit technical in places, it provides an excellent general introduction.
James, William. The Varieties of Religious Experience (*). Originally published in 1902 there are now various paperback editions available. This is the classic treatment, one of the first books on the psychology of religion and still read by scholars as an important treatment of the topic.
Shermer, Michael. How We Believe: The Search for God in an Age of Science. (*) Freeman, 2000 A provocative analysis of religious belief in its broadest domain. Probably the best starting point for a non-technical but informed discussion of religious belief.
Spilka, Bernard, Hood, Ralph, & Gorsuch, Richard L. The Psychology of Religion: An Empirical Approach. Prentice-Hall, 1985. Perhaps not quite as good as the Batson above, it is still a complete account of the psychology of religious beliefs and experience.