Necessary reading for any serious zen practicer. Note that there are three volumes in this series. Get all three. No regrets.
For the Zen student, this is a book of great value. The practice of working with the koan is clarified greatly by the Chinese master Hsu-Yun who, in a two-week retreat, gives daily Dharma-talks which detail the practice of the "Hua-Tou" or the "word-head," essentially "before-thought," in which a phrase or word such as "Mu" from a koan is used to trace each arising thought to the moment it arises, to the ground of no-thought. When this can be done in one's meditation, this is the gateway to the great freedom! Also valuable is the translation and commentary on the Diamond Cutter of Doubts Sutra. This is a good book to read after one has already been introduced and has personally begun a meditation practice in the Zen tradition. Highly recommended!