This is one of those books you keep forever, purely for its utility: it's packed with formulas, techniques, examples. But more than that, the authors lead you through the techniques and explain the concepts behind them, with  the goal of equipping you with the mental tools to attack any mathematical  problem you encounter. And to top it off, it's well-written, and the  "margin notes" provide some comic relief. The material is very  dense, and it's not a book I'd recommend for casual reading: this is stuff  you only work through if you're going to need it. But if you *are* going to  need it, this book will make it a lot more pleasant.