The manifesto of the communist party is not a huge theoretical foundational text.

If you're interested in marxist theory I would suggest reading modern Marxists like Slavoj Žižek. It's much more useful than reading actual marxist theory which is pretty dry and whose ideas have been adopted by various disciplines such as economics, history and philosophy already so they might not be as new and interesting as they were in the 19th century.

Maybe something like "Democracy in What State?" from 2011, you can just search for it on AA.