Here's an article that discusses a history of American Christianity. Special emphasis is given to the evolution of New England Puritanism to Unitarianism [the article].
The article makes me want to seek out more info on "history of American Christianity". This particular article does not delve into topics such as evolution to Quakers or Mormons. Perhaps there's a "family tree" of the growth of American types of Christianity?
While looking around for another text on this subject, I found this library search utility. Cool.
Update 7/20: Wikipedia's History of Christianity article