In fore-edge painting, artists paint images at the very edges of a book’s pages. The artwork is pretty much invisible when the book is closed until you fan out the pages just so, and then, boom: a fully realized painting appears as if by magic.
In the 1500s, a Venetian painter named Cesare Vecellio started painting on the edges of book pages, primarily portraits, but these were clearly visible when a book was closed. Hidden fore-edge painting was introduced by the British royal bookbinder Samuel Mearne in the 1600s.The practice spread around the world and peaked during the 18th and 19th centuries. It’s hard to know who painted what, and when, since artists typically didn’t sign their work. Plenty of the artwork has undoubtedly gone unseen, since most people wouldn’t notice the telltale shadowing on gold page edges that indicate the hidden artworks.
The University of Iowa has a great Tumblr page with lots of examples if you’re interested in seeing more.