Karl Marx and Friedrich Engels, The German Ideology selections in Tucker, ed., The Marx-Engels Reader. This is also available on-line, but I don't recommend printing it all out!!
Karl Marx, Eighteenth Brumaire of Louis Bonaparte (1852), at very least the sections in Tucker, ed., Marx-Engels Reader but you would be better served by getting the International Publishers edition (ISBN 978-0717800568). Do not get the Bibliobazaar edition (ISBN 978-1434615060)—it is a very poor and confusing translation. The full text, with helpful annotations, is also available on-line. If you have never studied the 1848 revolution in France, you may want to also consult the Encyclopedia of 1848 or maybe this chronology.
Louis Althusser, Essays in Ideology (1984).
Terry Eagleton, Ideology (1991).
David Forgacs, ed., An Antonio Gramsci Reader (1988).
Clifford Geertz, "Ideology as a Cultural System,"
in his The Interpretation of Cultures (1978),
as well as the other essays in this collection.
Eugene Genovese, Roll, Jordan, Roll: The World the Slaves Made (1974).
"Ideology and Fetishism"--short bibliography from my History and Psychoanalysis colloquium.
Frederic Jameson, The Political Unconscious (1981).
Karl Mannheim, Ideology and Utopia (1936).
David D. Roberts, "How Not to Think about Fascism and Ideology, Intellectual Antecedents and Historical Meaning,"
Journal of Contemporary History 35:2 ( 2000), 185-211.
Jacqueline Stevens, "Ideology and Social Structure. A Review Article," Comparative Studies in Society and History (1997), 401-409.
Eric Wolf, Envisioning Power: Ideologies of Dominance
and Crisis (1999)-chapter two provides a clear
and helpful overview.
Slavoj Zizek, The Sublime Object of Ideology (1990).