Just wanted to share this -
Beginning in the 16th century "fore-edge painting" on books was introduced. In the late eighteenth and early nineteenth centuries, fore-edge painting reached its height in England. This is how a fore-edge painting works. When the book is closed, you do not see the image because the gilding hides the painting. But, when you fan the pages to show the painting at its best and hold them between your fingers or in a display press, the colorful picture appears as if by magic. Info found at the Boston Public Library. Here are a couple of You-tube videos to demonstrate -
You have to admit, that is pretty wonderful!