Thanks for the explanation. I was not questioning the translation at all. It seems to me that since libellus is masculine, the "hic" and "est" and "quem" and "ille" could be referring to the book and not its author, just as "notus" could. I wondered if there were a specific grammatical point, and not just the context, that made the reference less ambiguous. Calpurnia's translation makes as much sense to me as the other.