Especially because so much secondary literature on Marx is terrible (unclear, obscurantist, barely bothering to quote the primary text), this book is a boon - the very opposite of all those flaws. If you're looking for a secondary text, and you need more than Jonathan Wolff's breezy introduction (Why Read Marx Today?) this is the first book to look at. One of the difficulties of reading Marx is that his ideas on any given subject are spread through a large number of books, essays, letters and notes; Wood's thematic arrangement makes this seem much more accesible, by giving you some idea of where Marx makes important statements on key topics.
As an undergraduate, I found that reading the relevant chapter or chapters of this book as my first step before addressing a topic was the most useful thing I could do. The book barely adresses any of the detail of the economics, being a philosophical examination, but chapter 15 'Dialectic in Capital' is nonetheless a very helpful introduction to what Marx is, and is not, trying to do.
His analysis of the topic of morality is perhaps a bit off key; but since, as always, he lays out so much original material, gathered from across the whole lifes' work of Marx and Engels, you can easily make your own mind up.
You can read the preface to this, the second edition, online at Wood's Stanford University homepage. It conveys something of Wood's obvious passionand sympathy for Marx, but also his keen critical sensibility - if he believes something is simply inconsistent, he says so, right out. Since there are no apolitical readings of Marx, let it be said that Wood's is a relatively libertarian, humanist one, and there's little Bolshevik, and no Stalinist inluence.