For an introduction to Marx's economic theory:
>From 1942, there are three classics: Sweezy's Theory of Capitalist
Development; Joan Robinson's Essay on Marxian Economics and the first part of Schumpeter's Capitalism, Socialism and Democracy. A few years earlier, there was Maurice Dobb's Political Economy and Capitalism. There is great stuff in each one of these books and Karl Korsch's chapters on Marx's economics in his Karl Marx are extremely helpful as well.
But...the recommendation I make is William J Blake, Marxian Economic Theory and Its Criticism, also published as An American Looks At Karl Marx. New York: Corden Press, 1939.
There are many reasons for this recommendation, but I will give only one: This is the only text which meticulously deals with the relationships between commodities, value, the derivation of the money form--that is, the conceptual foundations of Marx's theory. It is written in a very quirky style, but it shirks no difficult question and the bibliography is comprehehsive to that point in time.
Leo Huberman was a very good friend of Bill Blake's (so was Henryk Grossmann whose magnum opus I consider an epoch-making work) and Huberman's Man's Worldy Goods, from the middle of the Depression, provides a history of both capitalism and theory which is accessible and moving.
So these are the two recommendations--Blake and Huberman-- I would make from the pre WWII literature.
The third book I recommend with which to begin the study of Marx is
Geoffrey Kay, 1979. Economic Theory of the Working Class (dedicated to Bill Blake and I.I. Rubin, one of Stalin's many victims). It is a short book (130or so pp), written for the beginner, but the greatest care is given to defining precisely all the basic concepts.
Marx was a careful student of technology and the social implications of technological change; and his studies here are begininning to interest me more and more. There is a helpful essay on Marx as a student of technology in Nathan Rosenberg Exploring the Black Box; I find it interesting, stimulating, and thus recommend it. Studying Marx in relation to Charles Babbage and Abbot Usher, the latter a historian of technology, seems to me quite important.
As Marx's Theories of Surplus Value serves as a critique of the basic concepts of bourgeois economics, Alexander Gourvitch's Survey of Economic Theory on Technological Change and Employment is an extremely tightly argued and helpful review of the various theories of the employment and business cycle effects of technological change. Unfortunately it is very old (1940), but I find that without an introduction to the theories Gourvitch discusses present debates would be incomprehensible.
Marx's study of credit and finance may prove most decisive of all. That's why I am here--to read Doug, to see how he makes sense of Marx in relation to Keynes and Minsky and in relation to problems of debt at all levels, to the bank creation of money (lent to borrowers who seem expected to pay it back not through the sale of real goods but through more loans from other banks inventing money??!!), to the determination of interest rates, to the flows of speculative capital and the surrealistic world (as Doug puts it) of stock markets as forces of decapitalization. This is capitalism at its most dizzying and most fetishistic--the world of money, income yielding assets, credit and debt. And so one begins the study with Doug's Wall Street and looks forward to forthcoming issue of International Journal of Political Economy on credit money.
There has been important and very exciting discussion of Marx's work in terms of the philosophy of science. I think the best place to begin is Derek Sayer's Marx's Method: Science, ideology and Method. (there are several important books here but this is a good place to begin.)
And of course I have learned quite a bit from Paul Mattick Critique of Marcuse Marx and Keynes: limits of the mixed economy Economic Crisis and Crisis Theory Anti-Bolshevik Communism Economics, Politics and the Age of Inflation Marxism: last refuge of the bourgeoise
I do think the works I have recommended are accessible, compact in their individual chapters or sections (if not overall length), and very carefully and thoughtfully written. By those criteria, I have not mentioned several works which may presuppose a certain mastery but which elevate understanding to new heights. One can leave with a good understanding of what is Marx is saying (though each of these works is very controversial in its interpretation and thus must be compared to each other)and then make one's own critical assessment.
And, oh yes, read Marx. No one has yet brought so much in the way of theoretical clarity, historical understanding, philosophical acumen and a literary sense of the human drama to the the scientific study of society. He is also the best explainer of his own ideas.