I would posit the graph is showing the ephemeral popularity of books that either represent seminal landmarks in literature, philosophy, or other categories that

I would posit the graph is showing the ephemeral popularity of books that either represent seminal landmarks in literature, philosophy, or other categories that lend themselves well to the written word OR republications of books from the public domain for use in the classroom. For instance, several courses I took in Uni required books like Plato's Republic and Thomas Paine's Common Sense. Works that are in the public domain but were republished more recently.