In the book that rests on the knees of John the Baptist we make out the word CONSOLAMINI (take comfort) which begins a prophecy in the Book of Isaiah that was interpreted in the gospels as a reference to John the Baptist.
Please mention this source: The Ghent Altarpiece: panel John The Baptist detail, by Jan van Eyck; Hubert van Eyck, collection Sint-Baafskathedraal Gent, artinflanders.be, photo: Hugo Maertens, Contractuele beperkingen.