“A fore-edge painting is a technique of painting on the edges of the pages of a book. The artwork can only be seen when the pages are fanned, as seen in the animation below. When the book is closed, you don’t see the image because it is hidden by the gilding (i.e., the gold leaf applied to the edges of the page).”
The causes, appearances, and effects of the seasonal decay and decomposition of nature, 1837 by Robert Mudie
See and learn more about this fascinating art form at Twisted Sifter.
Beyond Survival in a School Library 
We have a book with this fore-edge painting. It’s a poetry book that is over my head. Not for reading. But it’s a lovely book – soft, good smelling leather and that lovely gilted artwork. Of a horse and DOGS!!
Lucky you! I would love to see one of these in person.