"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):
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.