Symbolism, Imagery, Allegory
Ezra Chater's opus is by all accounts worse than the poetry we wrote in middle school and wish we could erase from the Internet's long memory. The exact content of his book matters less than 1) its mockable badness, which allows Septimus to exercise his wit in making fun of it, and 2) the things added to the book later: Chater's inscription and the two letters to Septimus. These three ephemeral (a fancy word that means things that usually get thrown in the trash) texts survive by a fluke of fate: Byron borrowed the book and neglected to return it, and it just so happens that circumstance led to circumstance and the book fell into Bernard's hands.
Does the element of chance matter? After all, the letters survived, didn't they? True. But Arcadia suggests that the fragility of paper is also the fragility of history: if this long sequence of events hadn't happened just so, Bernard & Co. would never have known that a guy named Ezra Chater ever existed, much less that he may or may not have fought a duel with Byron. Sticking these key plot points into letters in a book that no one had any particular interest in keeping makes us wonder how much hasn't survived – and how much we can trust the documents that have.