Though it's not focused on economics and considers the entirety of 
Marx's system, the best single work I've ever read on Marx is Robert 
Heilbroner's Marxism: For and Against (1980).  It's sympathetic but 
not uncritical, and is especially good on both dialectics and the 
socialist conception of freedom.  I've assigned it for years to 
undergraduates to great effect.

Richie Adelstein