This paper aims at following the double thread which its title indicates : Le livre du diable . The latter is, at the same time, the author and the subject of the book which we watch taking shape during our reading through the use of a series of “different new processes, perhaps”. There are various clues which tell us that Bertrand was thinking of an object belonging to a new genre, rather than of a book which was to contain a text belonging to a new genre. On the other hand we wonder about Gaspard’s identity : he seems to be the devil, but he is, above all, the author of the text in which he theorises on the duality of art. Th e Fantaisies turn out to be an attempt at reconciling God and the devil, poetry and painting, in order to allow the birth of the “chiaroscuro” of a new genre.