Callahan (and the Austrian school in general) explain economics in terms of human action rather than the abstract and sometimes obtuse models of classical economists. Beginning with the simplest possible scenario, a single human acting in isolation, Callahan builds a hypothetical society and uses it to explain the crucial concepts of economics in a style and language that should be accessible to anyone who has completed high school.
He explains the concept of subjective valuation with his individual on the island, then begins adding people and concepts. He quickly takes us through direct exchange, a refutation of the labor theory of value, the introduction of money (including the explanation of the criteria that make something a good choice to use as money), time preference (and how the interest rate serves as the "price" of a time preference), and so on. In the second half of the book he explains concepts that are a bit more abstract - how do central banking and fiat money work? What causes the business cycle? How does a free market system handle externals (benefits or consequences imposed upon those not party to an exchange -e.g. water pollution).
Throughout it all, Callahan cogently makes the case for a truly free market as the only means of efficiently satisfying the desires of a society's members.
There are things I would have liked Callahan to cover better, for instance, a greater discussion of how the neo-classical economists work, and how their theories influence media reporting of economic issues (think about all the indicators that we are bombarded with in the business section of the paper). However, I don't see how he could have covered that material while keeping the book small and readable. He does give an extensive bibliography for those wishing to further investigate particular points.
A handy appendix gives the five page version of the history of the Austrian movement. It seems foolish to say this with it only being April, but I expect this will be the best book I read this year. I would give it more than 5 stars if I could.