I happened to be reading John Wilkins' introduction to "An Essay Towards A Real Character and a Philosophical Language" [1] yesterday, he mentions that Messalla Corvinus wrote a treatise on the letter S, though it seems we only have the title, "Liber de S. Lit[t]era" [2]. I wonder if Knuth had this in mind.

[1] https://archive.org/details/AnEssayTowardsARealCharacterAndA... , last paragraph, he also mentions other books on letters and parts of letters.

[2] http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:19...