While Teach Yourself Calculus is perfect for beginners who want to acquire a working knowledge of calculus, at the same time it is an excellent tool for anyone who wants to expand their knowledge beyond the basics. In a progressive, step-by-step fashion, the book builds from the ground up to offer comprehensive coverage of a range of more advanced topics such as multiple integrals. Each chapter features numerous worked examples and graded exercises.
"What is the book that changed your life? (not novels or religious texts)"
I think it's a wonderful way to connect in a less superficial way and learn a little more about each other. It's also a wonderful way to discover new books worth reading.
This is one of them. The full list is here.