Within the context of the ongoing storyline, Join My Cult! is the book Agent139 wrote to drive his doctors mad in the hospital.
I thought it was given away in the end when you see the effects on Dr. Fein, and it talks about Agent 139 taking his notes, but like most of the hints throughout there, I think on the whole it flew right under the radar.
The novel I'm working on now isn't written 'by' anyone, excepting short passages in first person. (Which are manned by any number of people, from 139 & 506 to a homeless guy on the street in Kansas City named Stevie.)