I know that most of you have grown up in this digital era, but me myself, I am a old school man, and, I usually use reference books. Today, I would like to recommend a small and practical book. Have you ever been working in an installation, in the middle of the Amazonian jungle or in the countryside with no internet connection, or, where your smartphone doesn't work? In this cases, I highly recommend you this pocket reference, plenty of useful information. From density of gases, up to computer ASCII and Alt+codes, through Math formulas or even how to do knots and bends... you can find anything you need in those moments when you have pressure, while your are working hard in a commissioning, you are in a rush and you can't remember, or you are stuck in a problem. So, buy it!... very cheap at Amazon. It perfect fits in your jacket pocket!