The Meditations by Marcus Aurelius http://classics.mit.edu/Antoninus/meditations.html http://classics.mit.edu/Antoninus/meditations.html I first read it when I

The Meditations by Marcus Aurelius http://classics.mit.edu/Antoninus/meditations.html http://classics.mit.edu/Antoninus/meditations.html I first read it when I was around 14. I think it helped me create a much more complete moral framework than anything I'd heard before (and probably since). Definitely my first introduction to Stoicism. It's not really a book, more of a collection of reminders to himself, which is probably why it's so useful and interesting.