Pipe Trades Pocket Manual
It contains literally everything you'd ever want to know about the mathmatics involved in pipefitting. I don't care WHERE you have to make that pipe go, this book will show you how to get it around every tight corner with ease. It is, without a doubt, one of the most practical books I've ever run across, and it will save you time on every job you do. If you work in the pipe trades, you will LOVE this book. And it's small enough to fit in your pocket!