Friedrich von Hugel (1852-1925). As important and indispensable as von Hugel's book may be, it is also very, very difficult. Take it from me, I gave this two volume doorstop a try, and finally had to throw in the towel. More my speed is Mysticism: A Study in the Nature and Development of Man's Spiritual Consciousness. First published in 1911, this volume by Evelyn Underhill remains the most read introduction to mysticism in the English language. Even Flannery, who had a general distrust of Anglicans, praised this "mine of information." (The Habit of Being, p. 116).