Only recommending M&E because they had a large influence on all Communist movements (obviously), and this was after the Manifesto, so you get to see a lot of the "newer" ideas taking hold (the ones that evolved into ML/"tankie" theory).
For understanding the Civil War from the perspective of the slaves, anything else is probably much better since Marx was interfacing mostly with American and English workers at the time. I will say that all of this ended up combining into ML over time though. The US Civil War had a massive influence on all theory that came after.