Those medieval people were seriously inventive when it came to getting the message out...
Fore edge paintings are the work of genius I think. I wish they would do those now, on modern books. Harry Potter with a fore edge? Oh yeah.
Some more links I found to fore edge paintings:
http://www.thisiscolossal.com/2013/09/fore-edge-book-paintings/
http://www.foredgefrost.co.uk/talks_foredge.htm
Fore Edge painting image search