The Book of Balance was a large book that was written by many Gray Jedi Masters over the course of the Gray Jedi. It taught the principles of why the Gray Jedi believed that the Force was not made up of a "light" and "Dark" side. It also explained the Gray Jedi's view on balance and what you must do to protect that balance. One of the requirements to become a Gray Jedi Jonin was to write an entry for the book. However in 387 ABY, that practice was disbanded and the book stayed as it was.