Although it is useful to examine the historical context of a philosopher’s writing, this is especially important for Karl Marx. Coming of age in the mid-19th century, at the height of the industrial revolution, he experienced first hand the squalid living and dehumanized working conditions of urban laborers. The injustices he found in capitalism led him to study the major French socialists--Fourier, Saint-Simon, and Proudhon; of equal if not greater importance was Hegel, whose concepts of alienation and dialectic Marx adapted: instead of Hegel’s idealistic, spiritual interpretation of history, Marx substituted a materialistic one, one that explained how societies progressed from slavery, feudalism, and capitalism to end, through a revolution of the proletariat, in communism. As for Hegel’s concept of alienation, Marx argued that when a worker is denied ownership of what he creates, and is forced–like an animal–into mindless, repetitious tasks, he loses not only money that belongs to him but also the pride of making something that reflects who he is. A final casualty of the factory is interpersonal relationships: formerly bound in solidarity with other craftsmen, he now views co-workers as slaves like himself, unworthy of respect or friendship. For Marx, alienation and exploitation are not overcome by interminable analysis but by action. This is expressed in one of his most memorable quotations: “'The philosophers have only interpreted the world in various ways; the point is, to change it.”
Marx wrote The Economic and Philosophical Manuscripts in 1844, although it was not published until 1932. Among his most philosophical works (as opposed to the later economic writings), it is significant for establishing Marxism’s essential humanism. If you do not have a copy, I recommend Erich Fromm's Marx’s Concept of Man. Continuum, 2004, available from amazon.com for $15.00 new, from $3.65 used; it includes The Manuscripts (107 pages) as well as other selected works, plus Fromm's excellent introduction. A free public domain copy is available here. Additional anthologies that include The Manuscripts are Karl Marx: Early Writings. Penguin Classics, 1992 ($11.32 new, from $2.65 used) and Writings of the Young Marx on Philosophy and Society, ed. and translated by Lloyd D. Easton and Kurt H. Guddat. Hackett Publishing Company, 1997 ($14.95 new, from $1.24 used).
The following resources provide commentary, lecture notes, and bibliographies.
Wikipedia: "Karl Marx", "Economic and Philosophic Manuscripts of 1844", "Influences on Karl Marx", "Marxist Philosophy", "Historical Materialism", "Dialectical Materialism", "Marx's Theory of Human Nature", "Marx's Theory of Alienation", "Class Struggle"
Stanford Encyclopedia of Philosophy: "Karl Marx"
"Encyclopedia of Marxism" (an extensive glossary of Marxist terms, produced by marxists.org)
Johan Van Der Hoeven, "Marx's Economic and Philosophic Manuscripts" in Karl Marx: The Roots of His Thought. Wedge Publishing Foundation/Toronto, 1976, pp. 66-86
Kaleb Shimp, “The Validity of Karl Marx’s Theory of Historical Materialism” Major Themes in Economics (Spring 2009)
Sean Sayers, “Creative Activity and Alienation in Hegel and Marx” Historical Materialism 11 no. 1 (2003): 107–128
Paul Blackledge, “Marxism and Ethics” International Socialism: A Quarterly Journal of Socialist Theory 120 (October[masked])
Patrick Coby, “The Utopian Vision of Karl Marx” Modern Age (Winter 1986): 22-32
Chris Buck, “The Nature of Marx’s Critique of Political Economy” (author is a Ph.D. candidate in Political Science at the University of Chicago)
Ada Albuquerque da Silva, “Redemptive Narratives in Marx and Nietzsche” intersections 10 no. 2 (2009):[masked]
Michael Hardt, “The Common in Communism” Seminaire (May 16, 2009) (argues that the forms of property have changed since the 19th century, requiring an adjustment in Marxist analysis)
G. William Domhoff, “A Critique of Marxism” (Professor of Sociology, University of California at Santa Cruz)
Ernest Van den Haag, “Marxism as Pseudo-Science” Reason Papers: A Journal of Interdisciplinary and Normative Studies no. 12 (Spring 1987)
“Karl Marx and Marxism” by Professor Larry L. Ping, Professor of European History, Southern Utah University
“Marx on Alienated Labor” by Professor Richard J. Arneson, Dept of Philosophy, University of California at San Diego
“Marx’s Naturalism and Theory of History” by Professor T. Manicas, Dept. of Philosophy, Queens College (CUNY)
“Historical Materialism: Karl Marx” by Professor Timothy R. Quigley, Dept of Philosophy at The New School
“The Sociology of Karl Marx” by Professor Frank Elwell, Dept. of Social and Behavioral Sciences, Rogers State University
"Cultural Theory: The Labor Theory of Value" (course lecture from Professor Ron Strickland, Dept. of English, Illinois State University
"Marxism Today 8: Alienation" (this gets good after two minutes into the video)