Value, Price and Profit was produced at a time when the labour theory of value had already matured in Marx’s brain. It was first delivered as a speech delivered by Marx to the International Working Men's Association (The First International) in June 1865, while he was working on the first volume of Capital that was published two years later.

The Preface of the Critique contains the first connected account of one of Marx's main theories: the materialist conception of history. The participants in history may not always be aware of what motives drive them, seeking instead to rationalise them in one way or another, but those motives exist and have a basis in the real world.

"There cannot remain any doubt but that the misery inflicted by the British on Hindostan is of an essentially different and infinitely more intensive kind than all Hindostan had to suffer before. I do not allude to European despotism, planted upon Asiatic despotism, by the British East India Company, forming a more monstrous combination than any of the divine monsters startling us in the Temple of Salsette. This is no distinctive feature of British Colonial rule, but only an imitation of the Dutch." (Marx)

Marx was asked in the summer of 1851 by Charles Anderson Dana, managing editor of the New York Tribune, to write a series of articles on the German Revolution. Founded in 1842 by Horace Greeley, the Tribune was the most influential paper in the United States at the time. These articles were written by Engels at the request of Marx, who was then busy with his economic studies and felt, besides, that he had not yet attained fluency in English. 

This work represents Marx's analysis of Napoleon III's coup d'etat of December 1851, which provides him with an opportunity for a meticulous examination of the ebb and flow of political forces and social classes in the period preceding Napoleon's seizure of power. Marx digs beneath the surfaces of political rhetoric and the manoeuvring for power by political personalities, and reveals the social forces and mechanisms at work during the political crisis. Its value, then, is as a class analysis of a political crisis.

"This work was Marx's first attempt, with the aid of his materialist conception, to explain a section of contemporary history from the given economic situation. Here the question was to demonstrate the inner causal connection in the course of a development which extended over some years, a development as critical, for the whole of Europe, as it was typical; that is, in accordance with the conception of the author, to trace political events back to the effects of what are, in the last resort, economic causes." (introduction by Engels)

"In the two revolutionary years of 1848-49 the League proved itself in two ways. First, its members everywhere involved themselves energetically in the movement and stood in the front ranks of the only decisively revolutionary class, the proletariat, in the press, on the barricades and on the battlefields. The League further proved itself in that its understanding of the movement, as expressed in the circulars issued by the Congresses and the Central Committee of 1847 and in the Manifesto of the Communist Party, has been shown to be the only correct one, and the expectations expressed in these documents have been completely fulfilled."

Wage Labour and Capital is based on lectures delivered by Marx at the German Workingmen’s Club of Brussels in 1847, that is, before the Communist Manifesto and at a time when Marx had not yet fully developed his theories of political economy. Although it is an early work, Wage Labour and Capital contains the outline of the Labour Theory of Value and many important insights into the workings of the capitalist system and the way in which workers are exploited.

The first Marxist FAQ, written by Friedrich Engels in 1847.

The Condition of the Working Class in England is a study of the industrial working class in Victorian England. Engels' first book, it was originally written in German as Die Lage der arbeitenden Klasse in England; an English translation was published in 1885. It was written during Engels' 1842–44 stay in Manchester, the city at the heart of the Industrial Revolution, and compiled from Engels' own observations and detailed contemporary reports. After their first meeting in 1844, Karl Marx read and was profoundly impressed by the book.

With Marx philosophy finally emerges out of the dark and airless cellar to which it was confined for centuries by scholastic thought and dragged out, blinking, into the light of day. Here at last thought is united with activity – not the one-sided purely intellectual activity of the scholar but real, sensuous human activity. 

Karl Marx understood very well the role of religion. He is often quoted as saying that religion is the “opium of the people”. But he said a lot more than that, and it was in this work that he shows a deep understanding of why people are religious. He explains that, to ask them to give up on their religious illusions, this means removing the conditions that require that illusion.