Probably The Prince, followed by Debt. For whatever reason, it broke my brain, and I stopped being intensely angry at rules breaking (by others or by myself). Growing up, I had this fairly common internal experience of viewing rules (often social rules, often implied) as being immutable moral truths. I would be furious if I had been taught a rule "Don't do X when Y". I would be both furious and distraught if someone did Y and was either not punished for it or even rewarded. Now it's very much tied to context and power. It's still frustrating when people are treated differently, but it doesn't keep me up at night in the same way.

I think this was also around the time I started watching Adam Curtis documentaries. Whatever else I think about him now, he does talk about power and society a lot, and the liberal confusion that occurs when they try to ignore power in their explanations of society.