This classic Zen Buddhist collection of 49 koans with commentary by
Mumon was originally published in 1934, and later included in Paul Reps
and Nyogen Senzaki's popular anthology Zen Flesh, Zen Bones.
Due to non-renewal it is currently in the public domain in the US (although
other parts of Zen Flesh, Zen Bones are not).
Koans are a Zen Buddhist spiritual technique which use word-play to achieve
enlightenment.
They frequently involve absurd or contradictory statements, which
are intended to create extreme cognitive dissonance in the mind of the pupil.
As such, their actual content or structure is not as important as the
mental state which they induce.
This collection includes some of the most famous koans, such as
Tozan's "three pounds of flax":
What is the Buddha? -- This flax weighs three pounds.