The non-short, relatively on topic list, in somthing near cronologic order:
Plato - The Republic, perhaps The Laws or On Virtue
Aristotle - Politics
Sun Tzu - Art of War
Suetonius - The Lives of the Twelve Caesers
Tacitus - *Histories, Annals
Thomas Aquinas- (i can't recall)
Machiavelli - The Prince
Thomas Hobbes - Leviathan
Rene Descartes - Principles of Philosophy
Leibniz, Spinoza - nothing specific, but good to have an idea of what their ideas were.
John Locke - Fundamental Constitutions of Carolina, *Treatises on Government
Hume - A Treatise on Human Nature
Rousseau - *The Social Contract
Kant - another guy that might be good to know about but nothing specific to read.
Adam Smith - An Inquiry into the Nature and Causes of the Wealth of Nations (like the book, the title is much longer than people let on, I recommend cliff notes

)
Hamilton, Madison, Jay - The Federalist Papers
Thomas Paine - *Common Sense, Rights of Man, *Age of Reason, The Crisis Papers
Thomas Jefferson - Virginia Statute for Religious Freedom, Declaration of Independence
Alexis de Tocqueville - *Democracy in America.
Thomas Robert Malthus - *An Essay on the Principle of Population
Marx/Engles - *The Communist Manifesto, Das Kapital
Nietzsche - The Will to Power (arranged by his sister post mortem, influencial with nazis)
Ricardo - Principles of Political Economy and taxation
Keynes - General theory of Employment, Interest and Money
Hayek - Road to Serfdom
Bertrand Russell - Wrote alot of stuff, pick something. "why i am not a christian" might be a good choice
Milton Freidman - Monetary History of the United States, Captialism And Freedom
I obviously missed alot (john stuart mill just popped to mind) but there are some good and some mediocre suggestions in here, and clearly still alot missing. * indicates a recomendation.
EDIT: Caeser, Marcus Arelious (sp?), more keep coming. could end up with a real long list if i wanted to.