Some good books...
1. Frederick von Hugel, Letters to a Niece (1928).
2. Aelred of Rievaulx, Spiritual Friendship (twelfth century, available in a 1974 translation by Mary Eugenia Laker, S. S. N. D.)
3. Francis de Sales (1567-1622, Catholic), Introduction to the Devout Life and Letters of Spiritual Direction (1988 translation by Peronne Marie Thibert)
4. Samuel Rutherford (a seventeenth-century Scottish pastor), Letters (available in an 1891 edition)
5. Kenneth Leech, Soul Friend
6. Martin Thornton, Spiritual Direction (1984)
7. Thomas Merton, Spiritual Direction and Meditation (1960)
8. Pierre Teilhard de Chardin (a French Jesuit), The Divine Milieu (1960)
9. Gerald May (a psychiatrist who "knows the difference between psychology and spirituality and disperses some of the fog that confuses them"), Care of Mind/Care of Spirit (1982)
10. Jerome Heufelder and Mary Coelho, editors, Writing on Spiritual Direction by Great Christian Masters (1982)
11. Francis W. Vanderwall, S. J., Spiritual Direction: An Invitation to Abundant Life (1981)
12. Douglas V. Steere (a Quaker), Together in Solitude
13. Ralph Harper, On Presence (1991)
14. Martin Luther, Letters of Spiritual Counsel (translated in a 1955 by Theodore Tappert)
A more extensive bibliography on spiritual direction is available at http://www.pastornet.net.au/jmm/spir/spir0015.htm.