I have recently finished reading THE CULTURE OF DISBELIEF: How American Law and Politics Trivialize Religious Devotion, by Stephen L. Carter (New York: Anchor, 1993, ISBN 0-385-47498-9). Even though the book is now over a decade old, I think it is still a great introduction to the religion-and-politics debate. And although it covers a wide range of legal issues, it is written for the layman.
Recommended reading.
