Fore-Edge Painting: an illustration that is hidden on the edge of the pages of the book. The technique allegedly dates back to the 1650s.
Shown here is a bible with biblical scenes of unknown provenance and a set of 4 scientific books on (and illustrating) the 4 seasons by Robert Mudie from 1837.
A famous bible used this technique to hide a painting of an orgy under the gilding of its edges!
Some modern books do this too, though it is certainly rare. The Cheese Monkeys by Chip Kidd (who is primarily a cover designer, not an author) employs this technique (without the gilding that hides the fact that content is there) in mass-print to say “GOOD IS DEAD” if you shift the pages one way and “DO YOU SEE?” if you shift the pages the other way.
This other books I posted HERE does the same thing to say YES one way, and NO the other.
