Dionysius the Areopagite, The Divine Names and The Mystical Theology, translated by C. E. Rolt, SPCK, 1979.

Dionysius the Areopagite was the sixth-century mystic
who exerted such a powerful influence on many
of the medieval mystics. These two short treatises are
lucid expositions of his thought.

Johnston, William, The Mysticism of "The Cloud of Unknowing": A Modern Interpretation, Abbey Press, 1975.

This analysis of the theology of The Cloud
of Unknowing
makes some very interesting comparisons to
Zen Buddhism.

Jones, Rufus M., Studies in Mystical Religion, Macmillan, 1909.

This is an engaging, if somewhat opinionated, survey
of mysticism from ancient times to the seventeenth
century, including several chapters on that of
the medieval period.

Kempe, Margery, The Book of Margery Kempe, translated by B. A. Windeatt, Penguin, 1985.

Margery Kempe (c. 1373-c. 1440) was an illiterate
woman from King's Lynn, England, who dictated the
story of her stormy life and her...