Prisoner (Tiresias Global Book 2)

Jack thought he was volunteering for some new weight loss and anti-aging research project. Instead, he finds himself trapped inside a mysterious empty room, with nothing but a chair and a video camera -- and a woman's voice, telling him what to do. She uses both reward and punishment to make him comply, as slowly, piece by piece, she has him reluctantly feminize and transform himself.

A male-to-female transgender transformation story.