 The book includes footnotes and letters that tell their own stories only vaguely related to the events in the main narrative of the book, and footnotes for fake books.Robert A. Heinlein's later books ("The Number of the Beast", "The Cat Who Walks Through Walls" and "To Sail Beyond the Sunset") propose the idea that every real universe is a fiction in another universe.This hypothesis enables many writers who are characters in the books to interact with their own creations.