Another older thing I have found useful is David Harvey: The Limits to Capital (U. of Chicago Press, 1982). Not easy reading, but it's hard to find thorough Marxist writings that are.
And of course a good Marx anthology is also useful, such as David McLellan's Karl Marx: Selected Writings (Oxford U. Press, 2nd ed. 2000).
Jon Johanning // jjohanning at igc.org __________________________________ After the Buddha died, people still kept pointing to his shadow in a cave for centuries—an enormous, dreadful shadow. God is dead: but the way people are, there may be, for millennia, caves in which his shadow is still pointed to. — And we — we must still overcome his shadow! —Friedrich Nietzsche