Accessible introductions
That Which Is Seen, and That Which Is Not Seen (html) by Frederic Bastiat
Economics in One Lesson by Henry Hazlitt
Thorough treatments
Man, Economy, and State with Power and Market (the Scholar’s Edition) [pdf] by Murray N. Rothbard (recommend reading with the study guide [pdf])
Human Action: A Treatise on Economics [pdf] by Ludwig von Mises (recommend reading with the study guide)
The Economics and Ethics of Private Property [pdf] by Hans-Hermann Hoppe
The Theory of Dynamic Efficiency [pdf] by Jesús Heurta de Soto
The Capitalist and the Entrepreneur [pdf] by Peter G. Klein
Critiques of popular approaches
Socialism: An Economic and Sociological Analysis by Ludwig von Mises
The Keynesian Episode by William H. Hutt
The Failure of the “New Economics”: An Analysis of the Keynesian Fallacies by Henry Hazlitt