Böhm-Bawerk's Criticism of Marx: Disputation by the Marxist author Rudolf Hilferding.
Marxian Economics: A survey from the History of Economic Thought website with bibliography and links.
Marxism: An article in the Concise Encyclopedia of Economics by David L. Prychitko.
Political Economy: Marx's writings on economics, reactions to Marx's economics and the Glossary of Political Economy from the Encyclopedia of Marxism.
Primer on Marxian Economics: A chapter from the Essential Principles of Economics provided by Drexel University.