Reddit mentions of Understanding Marxism

Understanding Marxism by Richard Wolff

Reddit is being weird, but yes I still have the titles of the books.

The Conquest of Bread by Peter Kropotkin, what I would consider to be a rather approachable primer on leftist philosophy. And it's available for free here.

Capital Vol. I by Karl Marx, heavy reading, there's three volumes and they are substantial books. But they are pretty foundational to the ideology of the movement. There are companion books written in modern times to help get to the point a little more quickly and succinctly. Such as those by David Harvey or Richard D. Wolff.

The Communist Manifesto by Karl Marx and Friedrich Engels, originally distributed as a pamphlet. Light reading and much more to the point than Capital. Available for free here.

Lastly, State and Revolution by Vladimir Lenin. For me this was a more historical book than political. The Bolshevik revolution was one of the only large-scale successful revolutions by socialists. And Russia, having been feudal before the revolution, is interesting. Not a big fan of Lenin, Trotsky, or Stalin though. They failed to actualize what the proletariat wanted and did some pretty nasty stuff along the way.

Other stuff I'd recommend: Anything by Mikhail Bakunin, On Anarchy by Noam Chomsky, The Origin of Family, Private Property, and the State by Friedrich Engels. Also, if you're interested in dissenting opinions, read Ayn Rand. Her magnum opus is very long and titled Atlas Shrugged. Her work has been very influential on modern capitalist and conservative ideology. Nonetheless, I believe it useful to contrast worldviews.