There have been some fine reviews already shared here applauding and outlining the importance of Underhill's masterwork on Mysticism. I couldn't agree more!
I disagree, however, with the suggestion that Underhill's section on Vitalism is outdated -- in fact such statements would seem to be themselves out of touch with modern times. The chapter on Vitalism offers a fascinating treatise on the root connection between mysticism and environmentalism. According to Underhill, Vitalism focuses on:
Freedom--Spontaneity -- the principle of a free spontaneous and creative life as the essence of reality--
The World of Becoming--Reality as dynamic--Life as incessant change
The mystic consciousness of reality two-fold--Being and Becoming--Transcendence and Immanence--
At its highest Vitalism may be conceived as "the universe flowering into deity"
These ideas are at the heart of modern day philosophical and ethical studies in eco-spirituality, ecosophy and deep ecology, as well as earth-based spirituality. Scientifically they inform and help to understand the Gaia Hypothesis (a scientific theory of the earth itself as a living organism). In terms of artistic creativity, to name just a couple of examples, the precepts of Vitalism are the heartbeat of modern earth works, and of the flowering of zen haiku and other contemporary nature poetry.
Underhill does not use such terms as eco-spirituality, etc, but her far reaching intellect, so profoundly alive in her essay on Vitalism, prepares the reader to make incredible leaps into all kinds of aesthetic, scientific and spiritual intersections that are so important today in understanding the new earth sciences, and in providing inspiration for environmental art, spirituality and the greening of ethical philosophies.
Evelyn Underhill is that most wonderful of combinations: a remarkably thorough student of mysticism, an intensely religious woman who had mystical tendencies herself, and a beautiful writer who can evoke the wonder of the mystical vision by the similes and metaphors she uses. An earlier reviewer thinkes her prose is "impenetrable." A strange evaluation, in my judgment, of a writer who is both poetic and pellucid. Here's a typical (and splendid!) example of her wordcraft: "By false desires and false thoughts man has built up for himself a false universe: as a mollusc, by the deliberate and persistent absorption of lime and rejection of all else, can build up for itself a hard shell which shuts it from the external world, and only represents in a distorted and unrecognisable form the ocean from which it was obtained. This hard and wholly unnutritious shell, this one-sided secretion of the surface-consciousness, makes as it were a little cave of illusion for each separate soul." (pp. 198-99)
It must be admitted that some parts of Underhill's classic haven't aged as well as others. Her chapters on mysticism and vitalism and mysticism and psychology, for example, are dated (especially the former). But her analysis in the second part of the book of the soul's journey to God, beginning with purgation and continuing through to mystical unification, still remains one of the best single treatments going. Underhill has a masterful grasp of western (Christian and, to a lesser extent, Jewish and Moslem) mysticism, and reading her just for the quotes from the great spiritual masters would be a delight in itself.
Don't sell her short. Doing so deprives you of the great pleasure of her company.