What is the book that has completely changed your way of thinking?

Someone suggested me "How to win friends & Infuence people". I read it 4 times in past 3 months. And I'll keep reading it through out my life. It has completely changed the way I see things. Now before I speak I think and I can understand how many mistakes I've been doing. It had a lot of impact on me. Like wise, are there any other books? Which book had such an impact on your way of looking at things?