On May 5th, 1818, Karl Marx, hero of the international proletatiat, was born. His revolution of Socialist theory reverberates throughout the world carries on to this day, in increasing magnitude. Every passing day, he is vindicated. His analysis of Capitalism, development of the theory of Scientific Socialism, and advancements on dialectics to become Dialectical Materialism, have all played a key role in the past century, and have remained ever-more relevant throughout.
He didn't always rock his famous beard, when he was younger he was clean shaven!
Some significant works:
Economic & Philosophic Manuscripts of 1844
The Eighteenth Brumaire of Louis Bonaparte
Critique of the Gotha Programme
Manifesto of the Communist Party (along with Engels)
And, of course, Capital Vol I-III
Interested in Marxism-Leninism, but don't know where to start? Check out my "Read Theory, Darn it!" introductory reading list!
Most books by Ha-joon Chang, especially 23 things they don't tell you about capitalism. Fun fact, Ha-joon Chang isn't a Marxist, he's a liberal, but his writing is still critical enough of capitalism that South Korea banned his books.
Anything on prolewiki's library are good reads as well.
For a broad look at the evolution modes of productions and how capitalism came to be there is this 60s textbook from the soviet academy of science, there even are a series of videos following the textbook by the finish bolshevik that is pretty good.