For those who want to explore more deeply, there is a wonderful collection of Koun Yamada’s teisho (Dharma commentaries) on the koans of the Blue Cliff Record (Hekiganroku) and the Book of Equanimity (Shoyoroku).
Yamada-roshi was my friend and teacher. If you meditate on these koans in sequence and then read his teisho, it is as if Yamada-Roshi is present in the room guiding you to discover your essential nature, just as he did in life.