A work of great poise, subtlety and erudition.
At the very outset, we are introduced to 4 frameworks of understanding reality:
a) Naturalism: that which is understood b y the senses
b) Idealism: that which is understood by thinking.
c) Philosophic scepticism: refuse to accept the realistic or idealistic answer to the riddle, contending there is no riddle to solve
d) Mysticism: the science of ultimates, the science of union with the absolute.
The book then dwells on the latter in its relationships with psychology, theology, symbolism and magic.
The main, second section of the book called 'The Mystic Way' traces the mystic's emergent journey - awakening, purification, illumination, voices and visions, introspection (recollection, quiet, contemplation), ecstasy and rapture, the dark night of the soul, and finally the Unitive Life.
The coverage is almost completely of the Christian mystical tradition, and hence misses out on many other traditions. That is not a shortcoming of the book, since I believe that the stages of the mystic way are tradition agnostic and universal.
For anyone who found Aurobindo's work a bit abstruse, this book provides similar scale and scope of coverage.