Dr. Friedman's books would also be a great introduction to libertarian thinking. See also the publications of the Ludwig von Mises Institute
I disagree with the premise that someone has to be a libertarian rather than a conservative, many people are both. And many of the best expositions of libertarian ideas have been written by economists, von Mises being one example.
I have read portions of The Theory of Money and Credit, and would read the rest if I had more time to pore over the details in it. It is a great book, and available online at The Library of Economics and Liberty
Neither Sowell nor Friedman were members of the Austrian School which seems to be a hallmark for Libertarianism.
However, the failure of both of those theories seems to be the tendency to ignore the political realities posed by democratic institutions.
We have a gigantic government because that is what the People thought they wanted and they had the votes to create it. Democracy will overrule proper policy on many occasions.