Great book that I used in "Great Powers," the basic thesis is that as Americans become more religious over time, they nonetheless know less about their faiths. So we believe more intensely even as we observe less and understand less. The "illiteracy" theme is so strong that I frequently refer to the book as "Religious Illiteracy."
That theme can get a bit tedious (How crucial is it really to know the Bible is all its arcane and conflicting imagery?), but what really marks this book as great is the short history of religion in the United States that is Chapters 3 and 4. Without those, the book would have been a waste of time in many ways, but with them, you get a history lesson that's worth the entire book's somewhat bitchy and condescending attitude.
So I say read the opening chapters that describe "The Problem" and most definitely read the two chapters (pp. 59-124) for the history (fascinating), but skip the proposal part that follows. The "dictionary of religious literacy" is a cool skim.