Favourite as 'will reread many times' or favourite as 'won't read again, but looked at life and society in a totally different manner after reading'?
Naissance de la Clinique: Une Archéologie du Regard Médical (The Birth of the Clinic: An Archaeology of Medical Perception)
Which is hard to prove is 100% non-fiction since it includes insights of Foucault. But look at a hospital, school, shopping mall, airport or ANY place where their could be heavy footfall.... and you realize that it's a panopticon. Without being dramatic (but explaining in a way US citizens will understand), a FEW police officers with rifles will have clear shots at anyone in that space.
As Foucault said - visibility is danger.
So not a fun read but you realize that your privacy was being destroyed even before CCTV, But it was initially designed for mental hospitals where it's arguable that it was required.... but they haven't changed the designs back SINCE CCTV,