Marx's most famous work, and his most substantial examination of capitalism, remains insightful and terribly useful to understand what in the heck is going on. Solid candidate for most important book of the 19th century, etc. Also: very long. Strap in.
Capital, prefaces26 Sep. 2017
There is, it turns out, a lot that Marx and Engels have to say before we actually start with the whole 'reading Capital' thing. In large part, Marx complaining about the state of political economy in 1870s Germany. This turns out to be surprisingly entertaining.
Capital, chapter 1: The Commodity26 Sep. 2017
Marx doesn't waste time: it's time to learn a theory of value.