Mysticism: A Study in Nature and Development of Spiritual Consciousness by Evelyn Underhill is available as a free audio book on LibriVox. It can be downloaded or listened to online.
Mysticism is a classic and comprehensive text on the experience of Christian mystics. It was published in 1911, when Underhill was only 36.
The first half of the book is introductory and more directed at an early-1900s audience. The second half is more timeless and I find these chapters especially illuminating and interesting.
As I have mentioned before, what she describes in the chapter on the dark night of the soul – what leads up to it and it’s characteristics – fits closely my own experience. When I read it a few years into the dark night, it was a great help in understanding what was happening, and also – perhaps ironically – a comfort. So although the other chapters resonate, the chapter on the dark night is what I appreciate the most.