There are myriad little introductions to Marx on the market: as the above reviewer noted, the Peter Singer "very short introduction," but also writings on his individual works, illustrated introductions, guides for the perplexed, Marx for Dummies, the Complete Idiot's Guide To..., etc.
For my money, the best introduction to Marx will always be the Communist Manifesto, but looking at my own notes from high school, it is clear that, unless you are willing to read hundreds of pages of Marx thereafter, and return repeatedly TO the Manifesto, there probably should be an "introduction to the introduction."
The difficulties in reading Marx on on several levels: 1) those adopted from the Hegelian line of German Idealism, 2) extremely complicated and foreign-seeming economic analysis, and 3) the integration of these within a PRAXIS and also an -ism, a tradition which would be variously elaborated by later "Marxists."
Now, after the Communist Manifesto, the best place to see this at work is in Engels' "Socialism Utopian and Scientific" and Rosa Luxemburg's "Reform or Revolution." What is important in all these works is their COMMUNIST orientation--they are not merely "theoretical" introductions.
So, those recommendations aside, this introduction is superior to many on the market because of its close analysis of the Marxist TEXT. There are ten close-readings of passages from Marx's career, which solves many of the problems for the reader approaching Marx: preconceptions and the inherent difficulties of the work. Preconceptions are rendered false problems by diving straight into questions that have NO relation to bogus bourgeois ideas of, say, the Soviet Union's collapse. And the difficult passages and concepts are excellently illuminated by Peter Osborne.
Marx is his own best introduction, but since he is ALSO the most misread author in history (after Nietzsche), perhaps The Communist Manifesto should be supplemented by this superb book. I also suggest the entries in this series for Lacan and Sartre.