A monk asked Chao-chou, "Does a dog have buddha nature or not?" Chao-chou said, "Mu". Mu is a Japanese word meaning "no", "not", "nothing" or "non-". That's all. And like this, Zen's most important "breakthrough" koan came to life.
The Book of Mu compiles contributions from forty teachers, both ancient and modern, to address the issue of that dog and its Buddhanature. Doesn't really matter if you are currently working on this koan, i.e. "striving to penetrate the Mu obstruction", or not–this is a terrific book. From the Introduction: "An expression of the boundless nature of all things and how that boundlessness interacts, informs, and completes the particularities of our lives". Maybe not for everyone. But definitely everyone. [NOTE: Love the dog on the cover, "Blizzard Dog" by Kate Hartland].