"Das Kapital" is a series of books by Karl Marx that critiques the capitalist production mode and explains how it gives way to the socialist production mode he advocated. Along with "The Communist Manifesto," the three volumes of "Das Kapital" are the key texts that formulate and explain the ideas of Marxism in great detail.
Marx started with Volume I, published in 1867, discussing the economic laws of the capitalist means of production. Marx died in 1883. After his death, his friend and collaborator, Friedrich Engels, compiled notes Marx had left to compile Volume II, subtitled "The Process of Circulation of Capital." The third volume, subtitled "The Process of Capitalist Production as a Whole," was published in 1894 based on Marx's notes.