Since you are starting with the Prince, I'm assuming you are looking for the works themselves, rather than secondary tomes. Contrary to what I like to write about these days, this is actually what I went to school for, so I can rattle off a nice long list for you. This is pretty much my half-remembered reading lists from college... And all in all would be a good start for an intro reading list to the evolution of western political thought. The ones I would REALLY recommend I've noted as Essential.

Discourses on Livy by Machiavelli (Much, much better than The Prince!) Essential

Leviathan by Hobbes

Second Treatise on Government by Locke Essential

Institutes of the Lawes of England by Cooke

The Social Contract by Rousseau Essential

Wealth of Nations by Smith

Democracy in America by de Tocqueville

On Liberty by Mill Essential

Civil Disobedience by Thoreau

What Is Property? by Proudhon Not at all essential! I just like it personally

The Communist Manifesto by Marx

Civilization and Its Discontents by Freud

Discipline and Punish by Foucault

Concept of the Political by Schmidt Essential

Man, the State, and War by Waltz

A Theory of Justice by Rawls Essential

I'd also say to throw some Hume, Kant and Nietzsche in there as well, but really I would just say geta reader for them and do excerpts. Plato's Republic and Aristotle's Politics also would be well placed there. All in all, this is a good mix of intro level works you would get for an undergraduate class on the evolution of Western Political Thought.

Some of the classics have already been listed, but I thought you might find the following hodge-podge of more recent works interesting as well. Plus, you might find them more readable. Some of these are straight philosophical works while others have a more historical methodology that still attempts to answer philosophical questions.

Giorgio Agamben, Homo Sacer: Sovereign Power and Bare Life and State of Exception. The former is much more dense than the latter, so it may be useful to have a study guide with you if you read it (good ones can be found all over the internet).

John Keane, The Life and Death of Democracy.

Karl Popper, The Open Society and its Enemies

Leszek Kolakowski, Main Currents of Marxism

Hannah Arendt, The Origins of Totalitarianism, On Revolution

Albert Camus, The Rebel

Francis Fukuyama's two-volume series on the history of the modern state: The Origins of Political Order: From Prehuman Times to the French Revolution and Political Order and Political Decay: From the Industrial Revolution to the Globalization of Democracy. Avoid his earlier book, The End of History and the Last Man.

Sovereignty in Fragments: The Past, Present and Future of a Contested Concept, edited by Quentin Skinner and Hent Kalmo.

Anything written by Charles Tilly

James C. Scott, Seeing Like A State: How Certain Schemes to Improve the Human Condition Have Failed and The Art of Not Being Governed: An Anarchist History of Upland Southeast Asia.

If you'd really like to dig into post-modern stuff, you'll soon realize that post-modernism rarely deals strictly with politics. Post-modernists tend to argue that society and culture has become increasingly politicized to the point where you can't disentangle the two.

If you're interested, you may want to check out:

The Postmodernism Reader: Foundational Texts, edited by Michael Drolet.

Stephen Eric Bronner, Modernism at the Barricades: Aesthetics, Politics, Utopia

Jean Baudrillard, The Gulf War Did Not Take Place

Finally, as a plug for my own topic of interest, you may like Mikkel Thorup's An Intellectual History of Terror: War, Violence, and the State. This is more than a history of terrorism. Thorup is a philosopher who looks at the history of terrorism and the state's response to get at a better understanding of the role of violence in shaping the modern era's political and social institutions.

Edit: On second thought, don't put yourself through the torture of reading Agamben's Homo Sacer. He is, though, one of the most prominent contemporary political philosophers. Instead, here is a clear, interesting animated summary of his ideas.

At what level? Are you looking for specific political philosophers, or introductory texts to gain a broader overview?