I've read The Catcher in the Rye by JD Salinger more times than I can count since I was 15--the first time I read it was actually the day Salinger died--it just made me feel so much less alone and weird than I thought I was prior and if you ever borrow my copy, you'll see several underlined sections like it's a study Bible.
I love dystopian fiction, 1984 by George Orwell and Brave New World by Aldous Huxley are life changing and eerie as hell when juxtaposed to the present.
Political philosophy is always interesting to read, even if it's at odds with your personal beliefs, but my favorite is The Discovery of Freedom: Man's Struggle Against Authority by Rose Wilder Lane, in which she argues that the role of an expanding government (the book was written in the 1940's, Lane herself burned her ration card during war time in protest of government assistance) is more harmful to the tenets of the foundation of America and the self-determination of the people than it is helpful, and backs up her work with historical events and ancient societies. She also gives a nod to Nebraska, my home state, which was, at the time she wrote the book, the only state in the union that had a balanced budget, and I just thought that was neat because when do you ever hear anything about Nebraska? As a native Nebraskan, the answer is darned near never.