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.
“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