Fore-edge paintings. Just look at them!
via Atlas Obscura
“While you don’t see them very often these days, fore-edge paintings were once some of the loveliest book illustrations around. Literally, they were around the edges of the book.
A fore-edge painting refers to an image painted or drawn on the closed leaves of a book. While covering the collected page edges in gold or silver leaf was a popular choice, sometimes artists went one step further and painted whole scenes and landscapes on them. This form of fore-edge decoration is known as ‘all-edge’ painting, and it was only the beginning.
Some ambitious, disappearing fore-edge paintings were painted on the inside edges of the pages, so that the hidden scenes could only be seen when the page block was fanned in a certain direction. If the book was simply closed, the page edges could look normal and unadorned (or possibly gilded), only revealing the image when one of the covers were shifted back, slanting the pages.”
–excepted from Eric Grundhauser, Atlas Obscura, September 29, 2016.