What is the sound of one hand clapping?

The book The Sound of the One Hand: 281 Zen Koans With Answers answers this as the first question. The book of ultimate spoilers.

According to Hoffmann, this Koan was composed by a Master Hakuin in the 18th century. It is the first Koan that a student is given when he enters a temple. The Koan cannot be explained by logic and can take up to three years to solve.

This is a bizarre book, giving rise to the question, why would anyone buy a book giving answers to questions that cannot be solved by logic? The phrase, "You're only cheating yourself" somehow springs to mind.

Link from Metafilter.