Michael Gilleland has a post on books he wished he owned. I'll copy his first one, J. D. Denniston, The Greek Particles, but here's the rest of my list.
Plato, Platonis Opera, vols. 1-5.
James Adam, The Republic of Plato, vols. 1-2.
John Burnet, Essays and Addresses.
Renford Bambrough, New Essays on Plato and Aristotle.
A. E. Taylor, Commentary on Plato's Timaeus.
St. Thomas Aquinas, Summa Theologica, vols. 1-5.
Charles Williams, Taliessin through Logres.
And since I don't want to be ungrateful, here's some books I'm thankful to own.
Charles Williams, Figure of Beatrice.
Moses Maimonides, The Guide of the Perplexed (trans. Pines), vols. 1-2.
Richard Purtill, Logic for Philosophers.
St. Thomas Aquinas, Summa contra gentiles, vols. 1-5.