In Order:
Stranger in a Strange Land - Robert A. Heinlein
The Fountainhead - Ayn Rand
Communist Manifesto - Karl Marx
Atlas Shrugged - Ayn Rand
Ender's Game - Orson Scott Card
The Divine Comedy - Dante Alighieri
Brave New World - Aldous Huxley
Mein Kampf - Adolf Hitler
Alas, Babylon - Pat Frank
Godel, Escher, and Bach - Douglas Hofstadter
This should give anyone a good look into the way humanity works, and you can truly look past any libertarian, communist, or religious. Lots of thinking and perspective. I've omitted many others have already suggested.