Lots of good stuff out there. I'll only repeat what others have said to underline the importance of a work.
Cosine:
I will start with some contemporary survey-level books that are good intros into the subject matter, the history, and are a smashing good read.
Intellectuals: From Marx and Tolstoy to Sartre and Chomsky, Paul M Johnson. Case studies of Rousseau, Shelley, Marx, Ibsen, Tolstoy, Hemingway, Bertrand Russell, Brecht, Sartre, Edmund Wilson, Victor Gollancz, Lillian Hellman, Cyril Connolly, Norman Mailer, James Baldwin, Kenneth Tynan, Noam Chomsky. A really good handbook to intellectuals, intellectualism, thier philosophies, and a wonderful way to inoculate you from the intellectual poseurs at university.
Modern Times Revised Edition: The World from the Twenties to the Nineties, Paul M Johnson. My copy has quite a bit of it underlined for reference.
A History of the American People, Paul M Johnson
PJ O'Rourke writes well and clearly. He has several good political book, some more serious than other.
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It is difficult to overestimate the influence of the Bible on Western political thought. A fine place to start. If you have ANY liking for Shakespeare, stick with the King James Version, which read the best if you of the appropriate reading ability.
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Also, the National Review List of 100 Best Books of hte 20th Century has quite a few on political philosophy:
http://www.nationalreview.com/100best/100_books.html----------
The Prince, Machiavelli
Discourses on Livy, Machiavelli
[For some tomes, such as The Prince, I highly suggest also reading the Cliffs Notes or a similar explanatory text side-by-side. In the case of The Prince, Machiavelli refers to (his) contemporary political scene. Not exactly a lot of household names, those.]
The Theory of Moral Sentiments, Adam Smith. Helps to dispel a lot philosophical horse manure generated in the last 150 years or so. Helps to understand his better-known work...
An Inquiry into the Nature and Causes of the Wealth of Nations, Adam Smith. THE text on market economics. So successful and so influential because Adam Smith understood human nature better than all the fakirs who followed.
Art of War, Sun Tzu (Avoid the commentaries, here. ST was the soul of brevity and clarity. 99% of commentaries are unhelpful)
Federalist Papers
Anti-Federalist Papers
Reflections on the Revolution in France. Edmund Burke. Made the connection between the French Revolution and Rousseau's writings...and tha tit would lead to totalitarian terror & a bloodbath.
Like TR wrote, Locke & Hume are very basic.
City of God, Augustine. Contains the very important, anti-totalitarian, anti-utopian message that we can never create Heaven on Earth. Very Platonic in outlook.
Thomas Aquinas. You can read his works, but a survey might be just the ticket.
Leviathan, Thomas Hobbes. Remember McRiley writing about the social contract? Well, here is where the idea comes from...
Capital, Karl Marx. If you read Sun Tzu, you realize you need to know your enemy. Truly, though, the 18th & 19th century German writers penned some of the world's most unpleasnat reads. There is a comic book version of Das Capital or Marxism that is a good place to start.
Friedrich Hayek produced several really good works on economics, knowledge dissemination, politics, etc. As with Marx, there is a comic book version of Road to Serfdom.
Jean-Jacques Rousseau. Find out where totalitarian philosophies got their genesis.
Common Sense, Rights of Man; Thomas Paine.
The Age of Reason, Paine. Here paine goes hammer & tongs against God & the Bible
