What is your favorite fictional (not real) book or book title that you read about in an actual book?

Sometimes authors invent books or book titles in a story to serve as plot devices, metaphorical representations, or even to improve the uniqueness of the world-building. Sometimes a fictional book or book title ends up becoming written as an actual book later on by the author or someone else.

Have you read Fillory and Further by Christopher Plover? Perhaps you have discovered great knowledge in the Encylopedia Galactica... or found enlightenment in the works of Princess Irulan? Whatever you do, don’t read the Necronomicon by Abdul al-Hazred, or for that matter, Aristide Torchia’s De Umbrarum Regni Novem Portis!