Evans-Wentz, W.Y., editor. (1958) Tibetan Yoga and Secret Doctrines. London, Oxford, NY: Oxford University Press.

Gardet, Louis and Olivier Lacombe. L'expérience du soi. Paris: Desclée de Brouwer, 1981.

Gardet, Louis. (1972) Etudes de philosophie et de Mystique comparées. Paris: Librairie philosophique J. Vrin.

Huang Po. (1958) The Zen Teaching of Huang Po. NY: Grove Press, Inc.

Izutsu, Toshihiko. (1971) The Concept and Reality of Existence. Tokyo: The Keio Institute of Cultural and Linguistic Studies.
_____(1982) Toward a Philosophy of Zen Buddhism. Boulder, CO: Prajna Press.

Kapleau, Philip. (1965) The Three Pillars of Zen. Boston: Beacon Press.

Kelsang Gyatso, Geshe. (1992) Clear Light of Bliss. Tharpa Publicatons: London.

Maritain, Jacques.  L'expérience mystique naturelle et le vide. In Etudes Carmélitaines, Paris, 1938 (23, vol. 2), p. 116-39. Also in: Quatre essais sur 1'esprit..., 1939. Translation: "The Natural Mystical Experience and the Void" in Understanding Mysticism, edited by Richard Woods. Garden City, New York: Image Books, 1980.

Sekida, Katsuki. (1977) Zen Training. NY: Weatherhill.

Suzuki, Daisetz Teitaro. (1971) Zen and Japanese Culture. Princeton, NJ: Princeton University Press.

Vishnudevananda, Swami. (1960) The Complete Illustrated Book of Yoga. The Julian Press, Inc.: NY. 

Many of these books are, unfortunately, out of print. Try your library's interlibrary loan system to see if they can borrow them for you.