I've read Machevelli's "The Prince" numerous times. "1984,"
I assigned the Prince at least once a year for years, I could practically receit it, but still reread it every time. A lot of wisdom in that book, once you undestand that Machiavelli isn't arguing for ammorality, but that there is a different moral standard for leaders as they will be judged by how well they protect the lives, liberty and territory of the people, above all else. I feel the same way about Mill's On Liberty, if you haven't read it you should, there's no clearer statement of American political values outside of the Declaration. Thing is though, I almost never through a book away. It's why no-one wants to help me move, 43 boxes at last count.

A lot of it just novels, but I will reread almost anything if its been a year or two. I can't count the number of times I've read the O'Brien Naval novels, or Tolkien or Hemmingway.
FQ13