The Demonomicon of Iggwilv

The name mostly says it all: this massive tome covers all sorts of subjects related to demons of the Abyss, all seemingly compiled by the same author (who is only ever credited as Iggwilv). The tome recounts both the oldest and most current profanities of the Abyss and demons, their strengths and weaknesses, and instructions for how to deal with each one should they be encountered. The breath of the information in the Demonomicon (and the often revolting nature of its contents) makes it quite a slog to read through-- although no doubt there are plenty of secrets to uncover if one dedicates the proper time...
The Demonomicon was discovered in the hidden study by Oliver, who currently has the tome in his possession.

Medium

Paper