The "Book of the People" was the most sacred text of the people of Yonada. According to Natira the Book was "to be opened and read" when they arrived at their "new world of the promise" on Daran IV. Before that time arrived, the Book was housed in a monolith in the chamber of Oracle of the People.
To open the monolith and behold the contents of the Book was considered sacrilege and warranted death. When one became a member of the people of Yonada, as Dr. Leonard McCoy did in 2268, he would be shown the Book, as well as submit to the insertion of the instrument of obedience.
It was later learned by a landing party from the USS Enterprise that the Book was merely a technical manual and guidebook. Yonada was, in fact, a multi-generational worldship and the "Oracle" its computer. By consulting the Book of the People, Captain James T. Kirk and Commander Spock were able to gain access to the computer and correct the flaw in its propulsion system, thus averting a collision with Daran V, and put Yonada on its proper course for Daran IV. (TOS episode: "For the World is Hollow and I Have Touched the Sky")