Plato's The Republic- One of Plato's great works and one that well portrayed platonic notions of the form of Good, the nature of Justice, and the proper order of society and government.
Plato's Gorgias- an interesting dialogue debating two opposing views of the good life, the life of virtue of the philosopher and the hedonist life of the orator.
Ideas have Consequences by Richard Weaver- A book that hypothesizes and analyzes the causes and course of the decline of western civilization, as well as speculate as to what is needed to fix the decline, and in the process makes the case for philosophic realism and a series of ethical and cultural principles.
As a Man Thinkith by James Allen- James Allen's key work for laying out his philosophy on how a person's mental state effects their success.
The Dhammapada, translated by Eknath Easwaran- A book detailing the key ideas of early Buddhist thought, relying largely on quotations from Siddhartha Gautama himself.
The Reformation by Owen Chadwick- A book analyzing the nature of the causes, course, and result of the protestant reformation.