When curious about some philosophical issue or author, you might want to check one of these general reference texts:
Paul Edwards, ed., The Encyclopedia of Philosophy. New York: Macmillan, 1967. See also the 1996 Supplement (Donald Borchert, ed. in chief) to it.
Edward Craig, ed., Routledge Encyclopedia of Philosophy. London: Routledge, 1998.
Robert Audi, ed., The Cambridge Dictionary of Philosophy (2nd ed.). Cambridge: Cambridge UP, 1999.
Ted Honderich, ed., The Oxford Companion to Philosophy. Oxford: Oxford UP, 1995.
Nicholas Bunnin and E.P. Tsui-James, eds., The Blackwell Companion to Philosophy. Oxford: Blackwell, 1996.
Simon Blackburn, The Oxford Dictionary of Philosophy. Oxford: Oxford UP, 1994.
Mario Augusto Bunge, Dictionary of Philosophy. Amherst (NY): Prometheus Books, 1999.
For more detailed information on specific historical periods, philosophical schools, or individual thinkers, you may want to refer to some of the following recently published companions:
The Blackwell Companions (there are several of them, dedicated to the different areas of philosophy: e.g, Aesthetics, African-American Philosophy, Analytic Philosophy, Applied Ethics, Bioethics, Business Ethics, Continental Philosophy, Cognitive Science, Environmental Philosophy, Epistemology, Ethics, Feminist Philosophy, Metaphysics, etc.)
The Cambridge Companions (mainly dedicated to individual philosophers: e.g., Aristotle, Aquinas, Augustine, Bacon, Descartes, Foucault, Gadamer, Hegel, Heidegger, Hobbes, Husserl, Levinas, Locke, Marx, Mill, etc.). Some of the Companions are dedicated to entire periods of the history of philosophy or to philosophical movements (e.g., Christian Ethics, Early Greek Philosophy, or Feminism in Philosophy).