The Mask of Appolitis is a book revealed in Dishonored: The Corroded Man.
[Extract from a lurid gothic novel, allegedly based on true events]
They came at once, out of the dark, out of the shadows! Silence was their skill, murder was their art! The history of the Empire was determined in that great room on that fateful night, the night the Mask and his Companion revealed themselves and saved the City of Appolitis with nothing but the force of their fists and the sharpness of their wits!
This extract is quoted at the beginning of Chapter 21.