An incomplete but useful list of classics might be the list following. You may wish to start with wikipedia entries on them, or other sources, but reading the texts themselves is useful!
Plato: The Republic
Aristotle: Nichomachean Ethics; and Politics
Thomas Hobbes: Leviathan John Locke: Two Treatises of Government
Rousseau: The Social Contract
Montesquieu: The Spirit of the Laws De Toqueville: Democracy in America
Karl Marx: The Communist Manifesto; and Das Kapital.
JS Mill: On Liberty (the classic Utilitarian Enlightenment work)
John Rawls: A Theory of Justice (A hugely important work on rights and principles of political decisionmaking)
Robert Nozick: Anarchy, State and Utopia (a right-wing / libertarian oriented response to Rawls)
Michael Sandel: Liberalism and the Limits of Justice (critique of liberalism and introducing communitarianism)