Anyone who browses book sales should acquaint themselves with the old technique of fore-edge painting:
"...the page block is fanned and an image applied to the stepped surface. If the page edges are themselves gilded or marbled, this results in the image disappearing when the book is relaxed. When refanned, the painting magically re-appears."I hadn't realized that the painting can be "double" in that a different image appears depending on which way the pages are fanned. The technique was widely used in the 19th century, so if you're at a booksale and find old books with gilt edges, fan the pages - the presence of a hidden fore-edge painting may substantially alter the value of the book.
The best single website on the subject is Martin Frost's UK site. There is a gallery at that site, and a separate gallery of what are termed "unorthodox" images that are VERY NSFW.