Just been reading some of Althusser's Lenin and Philosophy and Other Essays and it's essentially a book to say that Lenin was right to say that everyone needs to study Capital. It's good advice.

I remember watching the extra DVD that came with Lord of the Rings. There's a guy on the DVD who claimed to have read the series once a year every year and was thrilled to be asked to be involved with the movie. I wouldn't say Marxists need to do that with Capital (although David Harvey did and it worked out well for him) but it wouldn't be a bad idea to study it carefully at some point and then to revisit it often enough to keep it fresh.

That said, my recommendation to the OP is Harvey's Rebel Cities. Fantastic book and may motivate the reader to say, 'Damn, how do I think like that, too?' And then they'll have to and want to read Capital, too.