I've read many of Harvey's works before. The man has written celebrated works on social philosophy, urban geography, international relations, literary criticism, history and heterodox economics. Virtually all of his past works have something interesting to say, and he occupies a well-earned place as one of the most cited scholars in the modern world. Depending on what field of knowledge or academic literature you're interested in, Harvey probably has one book in particular for you!
However, if you want to become acquainted with Harvey's Marxian work as a whole, this book is the best place to start. Basically, this book is a general introduction to the Marxian perspective on global economics, social change, political hegemony, and democracy by way of Harvey. Interestingly, there have been many attempts over the past decade to create such introductions, ranging from Chris Harman's Zombie Capitalism: Global Crisis and the Relevance of Marx to Terry Eagleton's Why Marx Was Right the new edition of 'Marx's Capital': Fifth Edition by Ben Fine and Alfredo Saad-Filho, the Anti-capitalism a Marxist Introduction essay collection edited by Alfredo Saad-Filho, and the recent and excellent The Invisible Handcuffs of Capitalism: How Market Tyranny Stifles the Economy by Stunting Workers by Michael Perelman. Overall, I'd say that "The Enigma of Capitalism" is tied with Perelman's recent book as the best of these introductions. So, by reading this book, you'll be giving yourself an excellent introduction to both Harvey and Marxist thought in general.
I'll avoid going into the details of this book, but here's how you could summarize the content: Harvey uses the recent financial crisis to illustrate the structural contradictions of capitalism, capitalism's negative cultural effects, the evolutionary nature of capitalism, the insurmountable (from within capitalism itself) social problems that capitalism thrusts upon humanity, and the way that Marxism provides a conceptual scheme to understand and confront these problems. If this sounds overly technical, fear not- it's probably my fault. Harvey himself does a great job of connecting his theoretical understanding of capitalism to the actual concrete social and cultural problems the modern world faces. In doing so, he demonstrates Marxism's superiority to orthodox economics' stodgy obsession with creating perfect mathematical models while ignoring the sociological and ecological effects capitalism has on both mother earth and the greater mass of humanity. Harvey's ability to present both theory and practical observations makes him much more lucid than most political commentators of this sort.
Some commentators are so steeped in the "practical" to the point where they can't see the ideological impetus behind ecological and social problems. Meanwhile, other commentators seem to conceive of society's ills as nothing more than some sort of theoretical inefficiency, without taking into account the economic causal factors behind social and ecological ills. Harvey's approach, and the Marxist approach when done right, shows that you can't describe concrete problems without using theory, and you can't develop a meaningful theory without careful analysis of concrete problems. This should be a highly recommended read for intellectually curious and socially conscious readers.