John Witherspoon is one signers of the Declaration of Independence, and was a minister and president of Princeton Seminary. I'm not a big fan of his writings, but they are useful in marking the initial steps of distinctively American Presbyterian, as well as reflecting the thoughts of one of the U.S.A.'s "founding fathers."
Archive.org has kindly provided his works for easy download.
Also, for those unsatisfied by the above, please feel free to explore the other copies of the works available at Archive.org (link to unorganized list).
I also found the following set (apparently incomplete?) on Google: