They say; "Don't judge a book by its cover". I think I found why.
In really old books people painted illustrations on the edge of the book. An illustration you can only see when you slightly fan the pages of the book. Because when the book is closed you can't see any of it.
Take a look at this website with some images and videos showing them to you.
Makes me totally wonder why to put so much effort into something that people might not even see. Or would it be that it was custom back in those days to look at the book if it had a nice picture there instead of on the cover? Not judging the book by its cover, but by the edging?