Smith, Adam, Wealth of Nations (Great Minds Series), Prometheus Books, 1991, originally published in 1776.

This seminal work from the "Father of Economics" provides the first comprehensive discussion of economics in free-market societies. Though originally published in 1776, the book continues to be regarded as one of the most important economics texts in print.

Smith, Adam, The Theory of Moral Sentiments (Great Books in Philosophy), Prometheus Books, 2000, originally published in 1759.

This early Smith work—referenced twice in Freakonomics—focuses on the author's explorations of morality and ethics in humans. His arguments support the idea that morality is, in some sense, innate; this serves as an effective complement to what some perceive as the "every man for himself" view found in Wealth of Nations.

Becker, Gary S., and Guity Nashat Becker, The Economics of Life: From Baseball to Affirmative Action to Immigration, How Real-World Issues Affect Our Everyday Life, McGraw-Hill...