Fore-edge painting is 'a way of hiding a painting on the edge of book so that it can only be seen when the pages are fanned out.' Dating back to the 1650s, it is demonstrated in these wonderful GIFs showing the edge of an 1937 book titled Autumn. Some books even have double for-edge paintings, with different ones appearing depending on the directio you flip the pages!
One of our favorite sites, This is Colossal, got in touch with Coleen Theisen who originally shared a gif demonstration of the process, and was able to get her to send over several more. Colleen works with Special Collections & University Archives at the University of Iowa and you can follow their tumblr here.