Many of the already mentioned, plus a few more. I majored in American History at a Canadian university, where we were assigned as a textbook
The American Enlightenment: The Shaping of the American Experiment and a Free Society, a 700-page book edited by Adrienne Koch that contained the Federalist Papers and many other essays and letters by the Founding Fathers, especially John Adams, Thomas Jefferson, George Washington, Benjamin Franklin, and Alexander Hamilton. Students had to read at least some of the material and do class presentations and term papers based on it. I was fascinated by the high quality of thought in so many of the pieces, and wound up reading the entire book a couple of times. Prior to that I had had little direct exposure to the writing of any of the founders of the U.S.A. Reading their arguments helped me understand why the United States has been such a durable and successful democracy, and why other countries rarely imitate its constitutional institutions (because doing so would take too much power away from established elites). The book made a major impact on my thinking.
Two other biggies, IMHO: pretty much anything by V.S. Naipaul (both fiction and non-fiction, because he always writes to understand) and pretty much anything by Paul Johnson, although I'd particularly recommend Intellectuals and Modern Times.