Where is the best place to begin?

Evelyn Underhill was a prolific writer who published 39 books and more than 350 articles and reviews. In her early years, she wrote on mysticism; in her latter years on the spiritual life as lived by ordinary people. This latter work is more accessible than the earlier writing. Perhaps the best introduction to her is Letters, edited by Charles Williams, which give an introduction to her principal ideas, her style of guidance, and her immense sense of humanity and accessibility. The Spiritual Life, an edition of her BBC broadcast, first given orally, is a clear, concise introduction to her thought. Most of Evelyn Underhill's retreats were published, and many are still in print. A wonderful anthology of her work is Delroy Oberg's Evelyn Underhill: Daily Readings with a Modern Mystic.

Her key early works on mysticism include Mysticism: A Study in the Nature and Development of Man's Spiritual Consciousness and Practical Mysticism.