From Anonymous:
Ransomnote: Those are called fore-edge painting books.
Some more rare versions have paintings done both directions. One done with the pages bent in the jig one direction and a different painting done with the pages in the jig bent the opposite direction.
They have to be painted with a virtually dry brush.
foreedge.bpl.org
https://twistedsifter.com/2013/09/hidden-artworks-on-the-edges-of-books/
same as this link:
40 Hidden Artworks Painted on the Edges of Books ยป TwistedSifter
https://en.wikipedia.org/wiki/Fore-edge_painting
Fore-edge Paintings: Beauty on the Edge on AbeBooks
How to Add Your Own Hidden Fore-Edge Painting to Any Book (mymodernmet.com)
Some are also painted on the fore-edge without fanning them or gilding them.
(1) Pinterest
https://www.foreedgepainting.com/gallery
From Anonymous:
Ransomnote: Those are called fore-edge painting books.
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Interesting. I had never seen them before. ThankQ for the information and links. :)