Iain Dale put down some thoughts on his blog as to what the 10 key texts of Conservatism were. I was interested to see that several Conservatives tried to claim Liberal thinkers, such as JS Mill, as Conservative.
My own choice for Liberalism would be (in no particular order):
JS Mill: On Liberty
FA Hayek: The Constitution of Liberty
Karl Popper: The Open Society and its Enemies
John Locke: Second Treatise on Civil Government
Voltaire: Essay on the Manner and Spirit of Nations and on the Principal Occurrences in History
Rousseau: The Social Contract
von Humboldt: On the limits of State Action
Isaiah Berlin: Two concepts of Liberty
James Madision/Alexander Hamilton: The Federalist Papers
David Hume: A Treatise on Human Nature
Probably various others such as Aristotle's Politics or Joseph Steiglitz' Globalisation and its Discontents, should be included- please leave your ideas...