When the narrator reflects on his religious education with Father Flynn, he notes how the priest taught him about the existence of dense religious books, using a pair of similes in the process:
The duties of the priest towards the Eucharist and towards the secrecy of the confessional seemed so grave to me that I wondered how anybody had ever found in himself the courage to undertake them; and I was not surprised when he told me that the fathers of the Church had written books as thick as the Post Office Directory and as closely printed as the law notices in the newspaper, elucidating all these intricate questions.
The similes here—in which the narrator describes these unnamed religious books as being “as thick as the Post Office Directory” and “as closely printed as the law notices in the newspaper”—help readers understand why the young man feels so intimidated by the duties of a priest (such as reading such books).
It is notable that it is Father Flynn himself who uses these similes (and the narrator is merely passing on his descriptions), as they are not actually about the substance of the books but about their length and density. This is Joyce’s way of hinting at Father Flynn’s lack of integrity or commitment to his work—here, he is bragging to a child about the density of texts he has read rather than actually teaching the boy takeaways from said books.