Concrete Mathematics and calculus. I'd made it through several years of studying and applying calculus (either studying it properly in math courses or applying

Concrete Mathematics and calculus. I'd made it through several years of studying and applying calculus (either studying it properly in math courses or applying in physics and engineering courses) and never really grokked it. I could use it, but something important was missing in my understanding. I spent one summer working through this book with a professor, and suddenly calculus clicked for me. I still don't know what specific detail I had missed previously but I'll take it.