Flays Psychosis is a book on psychology.
Despite Flay's madness, Flays Psychosis is the one of the most important works on psychology. One of the things he talks about in this book is Emotion Resurgence. It is a concept where a sentient undead regains emotions lost when it gave up life energy as one of the living, and took in negative energy when it became an undead. This can result in insanity, including trying to end their existence, or in extreme cases, the beginning of being redeemed.