I used to own a lot of books. Then I started selling them off. Eventually I hope to get down to a short list of fifty books (or fewer!). Here's my start at the list, roughly grouped into categories...
Philosophy: I need my two-volume complete works of Aristotle, plus the Nicomachean Ethics in Greek and English (Terence Irwin's translation) in case I get around to rendering it into English. That's 4.
More Philosophy: I want to translate and compile a "best of Nietzsche" sometime. So I need my four-volume Nietzsche in German, plus various translations by Kaufmann and Hollingdale. +15 = 19.
Even More Philosophy: the Tao Te Ching, the works of Epicurus in English and Greek, "A Soviet Heretic" by Yevgeny Zamyatin, and "On Love" by Jose Ortega y Gasset. +5 = 24.
Reference: The two-volume, photo-reduced version of the Oxford English Dictionary, a more user-friendly English dictionary, The Synonym Finder, and dictionaries and grammars for Latin, ancient Greek, and German. +10 = 34.
History: "The Evolution of Civilizations" by Carroll Quigley, "Albion's Seed" by David Hackett Fischer, "The Anglosphere Challenge" by Jim Bennett. +3 = 37.
Fiction: "WE" by Yevgeny Zamyatin, "Anthem" by Ayn Rand, "Les Miserables" by Victor Hugo, "I Am David" by Anne Holm. +4 = 41.
Poetry: The complete poems of Walt Whitman, Langston Hughes, A.E. Housman, Sappho (Greek and English), Horace (Latin and English), Emily Dickinson, and Walter Kaufmann. +9 = 50.
Ta-da!