[UPDATED below.]
I figured that I would do this sort of thing once in a while, in order to chart a course of progress over time, to reminisce from points in the future, etc. This is a listing of books that "I'd take with me to a desert island" (a hypothetical that doesn't really tell you what would be most useful for you on a desert island, but rather illustrates something else, namely, the small number of books you'd want on hand over any others). It does not, at this point, include fiction, since I have yet to really explore that written world. Also, these are not synonymous with "favorite" books - though there is some overlap - but rather those books which I think I might come back to the most. Some I have not even gotten to reading but want to have them on hand as much as I can, just in case. It's not a perfect list; the point is to make it become more perfect over time. ;-)
David L. Norton, Personal Destinies
Alan Gewirth, Self-Fulfillment
Aristotle, Nicomachean Ethics
Viktor Frankl, Man's Search for Meaning and Man's Search for Ultimate Meaning
The Essential Plato
The Basic Writings of Kant
A Spinoza Reader
Wittgenstein, Philosophical Investigations
Basic Writings of Nietzsche and The Portable Nietzsche
A Kierkegaard Anthology
The Marx-Engels Reader
Robert Nozick, Anarchy, State, and Utopia
John Rawls, Justice as Fairness: A Restatement
The Essential Epicurus
Epictetus, The Art of Living
Marcus Aurelius, Meditations
Saint Augustine, Confessions
Erich Fromm, Man for Himself: An Inquiry into the Psychology of Ethics
Albert Ellis, A New Guide to Rational Living
The Analytic Theist: An Alvin Plantinga Reader
Thomas Jefferson, Writings
Benjamin Franklin, Writings
Thomas Paine, Collected Writings
Merriam-Webster's Collegiate Dictionary
Strunk and White, The Elements of Style
The Ayn Rand Lexicon and Letters of Ayn Rand
Leonard Peikoff, Understanding Objectivism
Alan Dershowitz, The Best Defense
Mises, Human Action: A Treatise on Economics
Hayek, Law, Legislation and Liberty
David Friedman, The Machinery of Freedom
Thomas Karier, Intellectual Capital
The Essential Jung
V.S. Ramachandran, Phantoms in the Brain: Probing the Mysteries of the Human Mind
Steven Pinker, How the Mind Works
Oliver Sacks, The Man Who Mistook His Wife for a Hat & Other Clinical Tales
Morton Hunt, The Story of Psychology
Jared Diamond, Guns, Germs and Steel
Douglas Hofstadter, Godel, Escher, Bach
Daniel Dennett, Darwin's Dangerous Idea
Noam Chomsky, Understanding Power
William James, The Varieties of Religious Experience
The New York Times Guide to Essential Knowledge
Michael Steinberg, The Symphony: A Listener's Guide
Alex Ross, The Rest is Noise: Listening to the Twentieth Century
Ursula Vaughan Williams, R.V.W.: A Biography of Ralph Vaughan Williams
Khalil Gibran, The Prophet
Mitch Albom, Tuesdays with Morrie
Jonathan Haidt, The Happiness Hypothesis
Nov. 4, 2012 additions:
The Jefferson Bible
Thoreau, Walden and Other Writings
Ruth Garrett Millikan, Language, Thought and Other Biological Categories
Henry Veatch, Rational Man: A Modern Interpretation of Aristotelian Ethics
Marquis de Sade, Justine, Philosophy in the Bedroom, and Other Writings
Walter Kaufmann, Critique of Religion and Philosophy
A. John Simmons, The Lockean Theory of Rights and On the Edge of Anarchy: Locke, Consent, and the Limits of Society
Charles Taylor, Hegel, Sources of the Self, and A Secular Age
Robert Nozick, Philosophical Explanations
Derek Parfit, Reasons and Persons and On What Matters