Really, you can't go wrong with primary sources.

Capitalism:

Wealth of Nations
The Fable of the Bees
Atlas Shrugged
The World is Flat
The Road to Serfdom
Capitalism and Freedom

Communism/Socialism:

Communist Manifesto
Das Kapital
What Is to Be Done?
The Prerequisites for Socialism and the Tasks of Social Democracy
Social Reform or Revolution
Chairman Mao's Little Red Book

And, as a general reference:

Oxford Dictionary of Politics
Oxford Dictionary of the Social Sciences
Oxford Dictionary of Philosophy
Oxford Companion to Philosophy

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Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?