Medieval marginalia is so well-known that amusing or disconcerting instances of it are fodder for viral aggregators such as Buzzfeed and Brainpickings, and the fascination with other readers’ reading is manifest in sites such as Melville’s Marginalia Online or Harvard’s online exhibit of marginalia from six personal libraries.
A story within a story (within a story?). The notes jotted down in the margins is often a private act, something that is profoundly for the self, however, Jones's mention of the variety of now-famous marginal notes (be it Medieval or otherwise) speaks to the act of annotation as a whole; an act for the self as one communicates with the text, that is made famous through publication, and that ultimately informs others as well.
As a side note, this reminds me of the book J.J. Adams and Doug Dorst published a while back, where one of the stories unfolds mainly through an endless conversation in the margins of a book.
http://www.amazon.com/S-J-Abrams/dp/0316201642