In this original and powerful work, John Roemer proposes a general theory of exploitation that provides a game-theoretic framework for expressing any conception of exploitation--feudal, capitalist, or socialist--in a standardized and explicit way, thus permitting a clear comparison of different ethical conceptions.
The book-in-itself isn't bad, but I'm not sure Roemer does anything here that Jerry Cohen doesn't do better. Roemer advocates for a game theoretic definition of exploitation in which a group (G1) is exploited if they there is an alternative in which G1 would be better off, another group (G2) would be worse off and G2 stands in a relationship of dominance to G1. This captures rather well the typical bourgeoisie/proletariat distinction.
However, the book itself tends more towards economic theory with formulaic proofs. For more philosophically and less economically inclined folk like myself, this means the argument loses some of its force since I honestly was just unable to follow his proofs a lot of the time. Jerry Cohen, conversely, provides much more philosophically inclined writings on the same issues. Cohen touches on issues of exploitation (extremely clearly and concisely) as well as class injustice, so there is not much done by Roemer than Cohen doesn't, in my opinion, do better for those who are philosophically inclined.
To sum, if one is wanting to learn of exploitation from an economical theory standpoint, I'm sure Roemer is a good source. However, if you are interested in exploitation from a philosophical standpoint, I reckon that Cohen can simply be read in lieu of Roemer.