Two techniques for analyzing free text documents: a bag of words approach based upon creating a TFIDF matrix, and an n-gram language model.
The Federalist papers were a series of essay written in 1787 and 1788 by Alexander Hamilton, James Madison, and John Jay (they were published anonymously at the time), that promoted the ratification of the U.S. Constitution. If you're curious, you can read more about them here: https://en.wikipedia.org/wiki/The_Federalist_Papers . They are a particularly interesting data set, because although the authorship of most of the essays has been long known since around the deaths of Hamilton and Madison, there was still some question about the authorship of certain articles into the 20th century.