The Ritual a piece of ceremonial magic by which the Other intends to free the Great and secret knowledge from whatever holds it captive...
-- Piranesi (Narration) (chapter 2)

Importance: This line of narration gives some insight into the presence of supernatural forces in the world of the novel. The Other's main goal is to uncover what Piranesi refers to as the 'Knowledge,' which appears to be a source of immense magical power. The Other also hopes to use magical rituals to help him uncover it. These ideas prime the reader to receive the novel's related supernatural plot points.

The closer 16 gets, the more dangerous Ketterley will become.
-- Laurence Arne-Sayles (chapter 3)

Importance: The Prophet—whom Piranesi later learns is Laurence Arne-Sayles—gives this warning while speaking to Piranesi. This conversation serves as an early warning sign of the Other's true, villainous nature. However, for the time being, Piranesi defaults to trusting the Other. The Other tries to manipulate and...