A fore-edge painting is a scene painted on the edges of the pages of a book; invisible when closed. The technique was employed by high end bookmakers throughout history, and in many cases, the hidden artworks went undiscovered for decades or even centuries after their creation. https://www.messynessychic.com/2019/11/05/there-might-be-a-secret-painting-hiding-in-that-old-book-of-yours