It depends on what you consider a book. While Leonardo Da Vinci’s Codex is probably the rarest manuscript in the world, the text is now widely available due to Bill Gates translating digital formatting. However, if you’re talking about rare stories - Homer, from the famous Iliad and Odyssey has come back into the public eye after incredibly rare clay tablets were discovered in Greece, with missing lines from The Odyssey (although we believe these stories were orally told).