Caesar's Commentaries, by Julius Gaius Caesar

Teaches that while technology will change and political systems will be distinct and nuanced, the human animal is just about the same.

When I see something that people did then that they still do today, I find that I can predict what a group of people is likely to do in a given situation in the future.

This both blows my mind as things "click" and makes me sad. In either case, I'm better prepared.