“One cannot write about Jonathan Strange and Mr. Norrell without considering the footnotes. The experienced reader is conditioned to see footnotes as dry, as a way of grounding the text in reality. But footnotes are also an intervention, or intrusion into the flow of the text, and Clarke takes advantage of this figuring. In Jonathan Strange and Mr. Norrell, it is in the footnotes that the world of the fantastic slips through to disrupt the meaning or common understanding of the tale told in the main text. The “explanation” they offer is of worlds slipping between each other, of uncontrolled contact with fairy.”
Book:Rhetorics of Fantasy