"Ricardo's difficult idea"
Paul Krugman
(paper for Manchester conference on free trade, March 1996)
http://web.mit.edu/krugman/www/ricardo.htm"
(Krugman is an economics professor at Princeton, formerly at MIT; you may recognize him as an op-ed columnist for the New York Times)

"David Ricardo and comparative advantage"
http://www.systemics.com/docs/ricardo/david.html
(A basic explanation; includes links to Ricardo's book)

_On the principles of political economy and taxation_
David Ricardo
3d edition 1821
link here
(This is the book in a 333-page pdf file -- approximately 1.2 megabytes)