The book provides an introduction to the subject of mysticism, presenting it from the point of view of metaphysics, psychology, and symbolism. It examines the different stages of development a mystic typically experiences.

Underhill emphasizes the practical, here-and-now nature of spiritual life. She argues that spirituality is a genuine and abiding human fact, and that any complete description of human life must find room for the spiritual factor, and for the religious life in which it finds expression.

Evelyn Underhill, the preeminent scholar of mysticism, wrote 3 novels in her youth, of which this is #3. Constance Tyrrel, a poor but literate woman works in a bookshop, suffers ennui, wonders if there is more to life than what she sees, invokes a ritual that she finds in a dusty old volume. Meanwhile, a disembodied spirit is consumed by a desire to know about the nature and content of the material world. It is drawn by Constance's call, where it appears as a column of dust. The two embark on adventures edifying to both and, incidentally to the reader. - Summary by Josh Mitteldorf