The Ceniza brothers
I know they were brothers who had a shop where they restored vintage books, but in reality who were they? When Corso first met them, one of the brothers shows him an engraving in the book that depicts the archer. Upon closer inspection, that archer looks suspiciously like both of the Ceniza brothers. They also seemed to know the secrets that books and their engravings contained and how it could be made possible to use them in order to conjure the devil himself, so therefore they forged one of the engravings.
But who were they? Why did they disappear at the end and leave their shop empty? It just all seemed very mysterious.