I, Pencil
For those of you who've either never read it, or perhaps never heard of it:
Arguably the most concise explanation of the power of the free market to give people what they need, in the most efficient fashion, and the why freedom is not only the best, but the only way, to meet those needs.
The Foundation for Economic Education, the publishers of I, Pencil, have a great library of economic literature available, with many publications available free of charge in HTML and PDF format. Check them out.