This is probably one of the most important Marxist texts you've never read and it's pretty short to boot. Supposedly Lenin could recite it from memory. I held a small study group using it a while ago but I was reminded of it listening to the latest episode of the Revolutions podcast which details how the SPD did exactly what Marx and Engels said not to do in this document.

Of course we see what Marx describes time and time again throughout history. I think it's even applicable to the events of 2020.