Aristotle - Nichomachean Ethics. I had spent a few teenage years figuring out my philosophy of life/morality/behaviour, read this at 17 and realised someone beat me to it
David Icke - The Biggest Secret: The Book That Will Change the World. I read this at 16, connotated tons of it and tried to get others to read it, didn't really work but I started to read conspiracy/alternative science books thereafter.