I recently looked at How it Works...The Computer, and this book is an equal match in terms of complexity. I'm really not sure that the books in this series are aimed at children, they really do go into an astonishingly high level of detail! In fact I found out that this book was used by Thames Valley Police driving school as a general guide and is still asked for today by driving schools.
How it Works - The Motor Car throws you straight in to a description of the motor car with a complicated, labelled diagram of an engine. Each individual part of the engine is then examined in further detail with more diagrams and description. Perhaps I should have read this book before I started learning to drive, as I had very little idea of how a car actually works.
I learned a lot from reading through this book. Some of the sections are rather difficult to follow, and you do need to concentrate. You also need to read it through from start to finish, you can't just dip in and out as all the information is closely related!
Although the book is quite old now, I'm guessing that a lot of the basics are still relevant today. Some things have moved on a lot though - a page on safety at the end of the book shows a car with no air bags throughout, and a back seat without seatbelts or head rests. The main safety features pointed out are those which are intended to prevent an accident in the first place, like good handling and visibility.
The book was also published well before computers were used in cars, which I think nowadays are probably even more important than the mechanical elements of the car. There is some speculation about future advances in the world of motor cars, for example a micro processor which selects the gears of the car automatically to save fuel, but generally the author of the book doesn't seem to foresee much change in the future of motoring.