"I don't believe the words float around in his head in some sort of genius fog, such that they can flow out without his knowing anything about where they are coming from."

Well, leave out the genius bit, but that's more or less how my mind deals with words.

A book I've always been fond of is "The Road to Xanadu: A Study in the Ways of the Imagination" by John Livingston Lowes, which is a study of how this kind of thing may have worked in the case of Coleridge when he was writing the Ancient Mariner, and dreaming Kubla Khan, the idea being that it was more or less as heric sceptically describes it.