Here is a collection of essays written about the Austrian Business Cycle Theory (ABCT) by various economists throughout the last century. I suggest reading them in order from top to bottom - they're not that long and provide a great introduction into the theory. Alternatively, here's a PDF that you may download containing the entire collection: Austrian Theory of the Trade Cycle.

Introduction: The Austrian Theory in Perspective by Roger W. Garrison

The "Austrian" Theory of the Trade Cycle by Ludwig von Mises

Money and the Business Cycle by Gottfried Haberler

Economic Depressions: Their Cause and Cure by Murray N. Rothbard

Can We Still Avoid Inflation? by Friedrich A. Hayek

The Austrian Theory: A Summary by Roger W. Garrison

These essays were sourced from the Mises Institute. The originals can be viewed here.