Books + Secret Art = Super Awesome
"There are two basic forms, including paintings on edges that have been fanned and edges that are closed; thus with the first instance a book edge must be fanned to see the painting and in the second the painting is on the closed edge itself and thus should not be fanned. A fanned painting is one that is not visible when the book is closed."
The fanned fore-edge paintings you see here come courtesy of Colleen Theise at the Special Collections & University Archives at the University of Iowa. The first one is from an 1837 book by Robert Mudie entitled Autumn.
"Fore-edge painting, which is believed to date back as early as the 1650s, is a way of hiding a painting on the edge of a book so that it can only be seen when the pages are fanned out. There are even books that have double fore-edge paintings, where a different image can be seen by flipping the book over and fanning the pages in the opposite direction."