The Unofficial LEGO Technic Builder's Guide (2012)
Clear overview of some engineering fundamentals, excellent illustrations, lots of LEGO history and knowledge.
This book explains how to reproduce a whole variety of engineering systems in LEGO. I especially appreciate that it doesn’t assume much prior knowledge – e.g. it explains what a transmission is, and why it’s important, before showing how to build it. I only have a vague grasp of vehicle machines, so that was useful to me. The illustrations are great as well; clear and thoughtfully coloured. Going back to transmissions: having the input/output axles and selected gears highlighted in different colours made it easy to follow.
There’s also a lot of historical information about old LEGO pieces, and their different properties.
I don’t have much LEGO right now, so I can’t see myself building anything from it – but I can imagine myself using it as a real-world engineering reference, if I want a quick primer on how something works.