NiccoloMachiavelli's Discourses On Livy is something of the modern world's first primer on how to successfully run a republic. While anything that is written so long ago is bound to become somewhat dated, it is amazing how much still has relevance today. This is a perfect text for anybody looking to seriously enter politics, even as a simple citizen exercising their franchise.
See http://www.constitution.org/mac/disclivy_.htm for HTML and text versions.