Human Action is the basis of the modern Austrian School. In 956 pages, Mises establishes the principals of praxeology; the science of human action. Thus established, Mises then logically creates a school of economic thought through one central axiom; man uses means to obtains ends. Simple, and clear.
In Man, Economy, and State, Rothbard rewrites Human Action as a very readable college-level textbook. In addition, Rothbard extends some of Mises' concepts, most notably the harm resulting from government interventions in the economy.
Hoppe, in Ethic of Private Property, wraps the previous two texts into an unassailable case for freedom.
These books are available at The Store on Mises.org. In addition, The Ludwig von Mises Institute offers Human Action and Man, Economy, and State as free PDF downloads. Enjoy all three, and understand the wonders of Liberty through private property.