What's the smartest thing you ever read?

hard to say, but probably God's Fury, England's Fire: A New History of the English Civil Wars, which was pretty good imo. There was also a trilogy about the history of the Seleucid empire, but i could only make it through half of the second book because the author kept sperging about dates every other paragraph. made it much more difficult to read than was strictly necessary.

Thermodynamics textbook junior year of my Engineering degree. Which was quite literally rocket science.