I'll go first.
Meditations by Marcus Aurelius.
It taught me about stoicism, how to live a life of discipline and sacrifice, and how we only have control over our thoughts and actions so that is what we should focus on. We can't control other people and their opinions so don't worry about them.
You can read my notes here if you'd like (sorry they're so long ๐):
https://www.twosapp.com/58c2f1a3...