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).

Some Internet resources are becoming more and more valuable. When building your philosophical knowledge, you may want to visit the following websites:

A Dictionary of Philosophical Terms and Names

EpistemeLinks.com

Stanford Encyclopedia of Philosophy

The Philosophical Lexicon.