This is a list of some of the many philosophical books that are now in the public domain and can be browsed or downloaded (in PDF format) from Google’s archive. You can find more by searching at http://books.google.com/ and selecting “Full view books.”
David Hilbert, The Foundations of Geometry, translated by E. J. Townsend (1902)
Bertrand Russell, The Principles of Mathematics (1903)
Bertrand Russell, An Essay on the Foundations of Geometry (1897)
Albert Einstein, Geometrie und Erfahrung (1921)
Gottlob Jäsche, Immanuel Kants Logik: Ein Handbuch zu Vorlesungen (1800)
Alexander Bain, Logic: Deductive and Inductive (1887)
Hermann Lotze, Logic, Vol. I, translated by Bernard Bosanquet (1888)
Hermann Lotze, Logic, Vol. II, translated by Bernard Bosanquet (1888)
Christoph Sigwart, Logic, Vol. I, translated by Helen Dendy (1895)
John Stuart Mill, The Subjection of Women (1870)
John Stuart Mill, Utilitarianism (1887)
Henry Sidgwick, The Methods of Ethics, 4th ed. (1890)
John Stuart Mill, Auguste Comte and Positivism, 4th ed. (1891)
F. H. Bradley, Principles of Logic (1883)
James Adam, The Republic of Plato (1902)