The Corroded Mind, by Álvaro Cardoza is a book found in Death of the Outsider.
Effects of the Outsider's Influence
The anatomical evidence clearly demonstrates that the mind itself deteriorates under the Outsider's influence. This conclusion is inescapable when you examine the behavior of his followers. Observe the case of a young woman, devoted to the Strictures and pure by all accounts, who fell into corruption by mere belief in the Outsider. There was no vice she would not stoop to: the slaughter of animals for superstitious rituals, wishing harm to others through heretical spells and curses, desecration of texts with profane writing. Her Errant Mind led to Rampant Flesh and a Lying Tongue. Soon, she abandoned every guiding principle and ran away in the night. Her abused and gravid body was donated to the Abbey two years later. Autopsy revealed deterioration of the connecting lobal tissue. Her mind was visibly divided, corroded beyond all repair, all from belief in the Outsider.
This book can be found in Teresia Cienfuegos's house in Upper Cyria District during the mission Follow the Ink. It can also be found next to a sleeping Overseer on the first floor of the Royal Conservatory during the mission The Stolen Archive.