Mountain's Twin was a Journey book in Jagang's possession which Nathan wanted to destroy. It was the twin journey book to Joseph Ander's journey book from the time of the Great War.
In Soul of the Fire, Dalton Campbell reflects on how an official was arrested for stealing a book written by Joseph Ander. Reportedly, the official was drunk, and the book was found in his pocket.
Richard Rahl was given the "original" version in Anderith by Zedd in the form of a raven, which documented how Joseph Ander banished the Chimes.