The article contains numerous photos and some videos. Please make sure you visit the site to see them all, and to learn further details about the books. Thanks QoS, I found it fascinating!
Here's a sample:
The first two photos are from the University of Iowa Library, the others were courtesy of Boston Public Library and following photo is courtesy of Johns Hopkins Library. It's a bible with split fore-edge painting:
Link to article: http://twistedsifter.com/2013/09/hidden-artworks-on-the-edges-of-books/