The characters within a book were, from a certain point of view, identical on some fundamental level ‒ there weren't any images of them, no physical tangibility whatsoever. They were pictures in the reader's head, constructs of imagination and ideas, given shape by the writer's work and skill and the reader's imagination. Parents, of a sort.
Books and drafts mean something quite different for different thinkers. One collects in a book the lights he was able to steal and carry home swiftly out of the rays of some insight that suddenly dawned on him, while another thinker offers us nothing but shadows - images in black and grey of what had built up in his soul the day before.