Despite the explosion of Marxist economics, I think very few works can be said to be based strictly on Marx's own labor value-theoretic foundations which, needless to say, Mattick did not see destroyed by the so called transformation problem that has excercised economists. Grossmann's reconstruction of Marx's crisis theory is one such strictly Marxian work; Mattick's critique of the Keynesian interventionist state is another. Both theories were soon after their publication massively confirmed by events: the world was thrust into Depression only months after the publication of Grossmann's magnum opus in 1929; Keynesianism went up in stagflationary ashes only a few years after the publication of Mattick's magnum opus (1969).

As for an introduction to Marx's basic concepts there are two other important works: William J Blakes Elements of Marxian Economics and Its Criticism (1939) and Karl Korsch's Karl Marx (1938). In my opinion, no serious student of Marx should be without these two works, as well as the contributions by Grossmann and Mattick.

These works develop a Marxian theoretical perspective differnet than the one presented in Anglo American Marxism by Dobb, Sweezy and Meek.

I also cannot more highly recommend each and every other work by Mattick: Critique of Marcuse, Economic Crisis and Crisis Theory,
Economics, Politics and the Age of Inflation, and Marxism: last refuge of the bourgeoisie?

Developing Marxism is helped by taking over the major theoretical clarifications and accomplishments of the past. And Mattick's work remains just that important, I believe.