Three Pillars of Zen, read when I was 16, it felt like "This is it. This is how I see it."

Few books by Ken Wilber, gave me a framework, a meta-frame of sorts when I was young.

Introduction to NLP (esp. chapter V on Meta Model, took me two years to process it)

Structure of Magic, vol. II - communication, incongruity

Mind and Nature by Gregory Bateson

And I could go on and on... but the winner is...

"Embracing Our Selves" by Sidra & Hal Stone.

This book and perspective changed my life completely. It was like a grenade bursting inside my mind and I've seen many people reacting to it this way (incl. people closest to my heart). It's a perspective shift, an insight. You've been warned. :)