Robert Paul Wolff always worth. Should read his Understanding Marx. He surprisingly says that Analytical Marxism has taken over Marxism. To get to Marxism he had to learn economics, the classical political economy of the Ricardian kind, and learn linear algebra. Hegelianism was not really central to the message (in my view, but he does say he hates Hegel). Labor Theory of Value explained, but he does not discuss Piero Sraffa's contributions directly here. There is no way to understand Marx without Sraffa. As he said in a paper in Social Research (subscription required) Sraffa "rescued [Smith, Ricardo, and Marx] from the trash heap of antique, outmoded ideas... [and provided] a serious alternative to marginalist theories."