Head and Heart: American Christianities (hc) by Gary Wills for 6 dollars.
I already read this a few years ago when I checked it out from the library, but it's such an excellent resource that I couldn't resist getting a copy for my personal collection.
See below for Wills's Fora.tv discussion of his book (which is about the history and conflict of evangelical and enlightened Christianity in America.) You can click the little button on the bottom right to get to the whole program.
The Philosophy of the Midlife Crisis
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