What is the word play in To Root of Mystery Clever Hans?

You're likely referring to the book "The Root of the Mystery: The Life and Legacy of Clever Hans" by Mark H. Denny. The wordplay in this book's title is a double entendre:

1. "The Root of the Mystery": This phrase literally refers to the root cause of the mystery surrounding Clever Hans, the horse that appeared to be able to perform arithmetic and answer questions.

2. "To Root of Mystery": This phrase, with the infinitive "to" added, suggests the idea of "going to the root of the mystery," implying a journey or investigation into the truth.

This playful use of the word "root" adds a layer of intrigue to the title and hints at the book's exploration of the complex and fascinating story of Clever Hans.