The contribution of Marx and Engels is essential to an understanding of a libertarian communist politics. It is a tool for the working class to critique capitalism, and to analyse class composition for future struggles. Here are the pdfs available of his major works, (Michael Heinrich’s introduction is a good place to start):
heinrich-michael-an-introduction-to-the-three-volumes-of-karl-marxs-capital
[Karl_Marx]_Marx_Later_Political_Writings_(Cambri(BookFi).compressed
Marx – Dispatches for the New York Tribune
Capital – (Penguin Classics) Volume II
Capital – (Penguin Classics) Volume III