Enjoyed reading this essay on how the technique of using footnotes in fiction has evolved over the years. Another example is Book: A Novel by Robert Grudin, a work of pure metafiction in which the footnotes try for a while to dominate and take over the main narrative.

open-book-library-education-read-159621[1].jpeg“In fact, what all of these works show—from Nabokov and Wallace to Danielewski and Boully—is that experimentation quickly stops being experimental when it works well, and gives way to progression … Footnotes, once the hallmark of pedantry and pretension, have now entered the realm of craft. More than a trick, footnotes can be technique. We’ve seen how they can be used to comment on a narrative or to create a new one, to overlap separate narratives, to evoke character in new ways, and to dig into difficult parts of who we are. Footnotes, in other words, no longer merely support a story; now, they can be the story.”
Jonathan Russell Clark, ON THE FINE ART OF THE FOOTNOTE

Source: http://lithub.com/the-fine-art-of-the-footnote/