Is there a documentary or book that really changed the way you thought about something?

I was brought up in a religious environment and was a Christian until about my mid 20s, then I met Omar Khayyam and learned that it was possible to be spiritual, even mystical, without the bureaucratic straitjacket of formal religion. I loved Fitzgerald's five translations but have since discovered Richard le Gallienne's paraphrase.

But before I met Omar I read The Screwtape Letters  by C.S.Lewis which is a real eye-opener for anybody with an interest in how their mind works and the tricks the subconscious can play with the way we do things. He may not have intended it that way but it proved to be a very useful tool.