Elster writes well and clearly and frequently suggests controversial but insightful readings of Marx. That said, there's not of Marx left when he's finished, and I think that is more Elster's fault than Marx's. Elster applies a "methodological" principle called methodological individualism, which he does not explain very well, and which is entirely antithetical to Marx's holistic ("dialectical") manner of analysis. Elster, in the name of wanting to explain the micromechanisms whereby things occur -- a laudable goal and one where Marx often falls down -- insist that social phenomena be explained by reference to individual and their properties individualistically described. It is doubtful that this is coherent, since many of the constitutive properties of individuals -- their class position, to take one example -- are inherently social. In general Elster, in this longish book, almost invariable takes as his target the less plausible and less sympathetic readings of Marx to attack, which makes his job easier but his attempt to make sense of Marx more quetionable.
Analytical Marxism -- the now largly moribund movement of which Elster was a founder -- has a lot to offer the understanding of Marx, but like some its advocates, Elster went overboard in getting rid of too much of Marx that did not fit his preconceptions -- many of which Marx himself criticizes without adequate recognition or response from Elster. Still, the book is important for serious Marx scholars. General readers might start with Elster's shorter version summarizing Elster's main conclusions -- and complement it with a more sympathetic though no less analytical book like Richard Schmitt's Introduction To Marx and Engels. (Schmitt does not think of himself as an Analytical Marxist.)