Since HN originally pointed me to Discworld, I thought I'd ask -- what other books -- fiction or otherwise -- have changed your perspective on human psychology / sociology / the space of possible minds?
Deliberate Calm is another one that helped understand on a more scientific level now our emotions are created and how that leads to our behavior.
The Prince (Machiavelli)
Beyond Good And Evil (Nietzche)
A History of Western Philosophy (Russel)
The Fourth Turning (Strauss, Howe)
Also, Discourses on the First Decade of Titus Livius (Machiavelli) is a fascinating read on human political machinations (albeit fairly slanted by Machiavelli's personal views)