I found this to be bleak and depressing. Graphic at times, but I would not consider it a thriller. A story was told, there was nothing to figure out. The last third was a different book and didn't fit with the rest. I get that the author was trying to be unpredictable, but you could have said Santa Claus was the killer and the reaction would have been the same.

Why the big focus on prison life? What did that add that a short paragraph couldn't have added???