MetaFilter has a short but amazing post about fore-edge painting, an entire genre of book illustration that's new to me. (Pictured is the fore-edge of a 1908 edition of The Wind in the Willows.) From one of the links:
Fore-edge paintings are watercolour decorations, painted on the ends of the pages of the fore-edge of a book. In most cases, a fore-edge painting is only visible when the pages are fanned out, expanding the panel formed by front edges of the book and exposing the painting.Link
The earliest records of fore-edge paintings date back to the 10th century, although the art did not flourish until the late 1600s. Fore-edge paintings are still being produced today and have evolved through the centuries to offer a complex and elaborate variety of paintings. In addition to the standard painting, sometimes known as a 'single', there are split, panoramic and double variations. Split paintings are those in which each half of fore edge bears a different painting. In 'Panorama' types the illustration covers not only the fore edge but the top and bottom edges of the book as well, giving a near-360-degree panoramic view. The 'double' type is amongst the rarest and most collectable. It shows an illustration when the fore edge is fanned out the usual way, and a completely different painting when the pages are fanned out in the opposite direction.