The Purpose of Jonah's Story

The end of the book left me questioning what Jonah's "goal" for writing the book was. Throughout the book I wasn't sure what the purpose of the story was, if it wasn't a story about the day the atomic bomb was dropped. In the last chapter, Jonah comes across Bokonon sitting on the side of the road, and he asks Bokonon what he's thinking. Bokonon reveals his idea for the last sentence for The Books of Bokonon, which happen to conclude Cat's Cradle. This gives the reader the impression that after Jonah talked to Bokonon, he did exactly what Bokonon said, which means that the book Jonah is writing is a history of human stupidity. After he finished writing, he committed suicide by eating ice nine and mocking "You Know Who". So is the Jonah's purpose for the book to be a history of human stupidity? 

Another concept is that the point the book was the same from the beginning, which was a book about the end of the world. Jonah begins his book by saying his original intention was to write a book about the day the atomic bomb was dropped, titled The Day the World Ended. By the end of the book, it seems like he did write a book about the end of the world. The scene being described after the ice nine was released seemed post apocalyptic, with little signs of life-- essentially the end of the world.

The issue with these ideas is the fact that the pretext says the nothing in the book is true. But does that mean the pretext also isn't true? What the reader believes is and isn't true in Cat's Cradle depends on his interpretation of the story.