Quite possibly my most popular question. For those looking for a basic introduction, How Cars Work is a good, cheap book for learning the fundamentals. If you’re like me, however, it won’t quite satisfy your appetite for knowledge, so I recommend going with more detailed and technical books, including Automotive Technology – A Systems Approach (Great, technical, in-depth with excellent diagrams) and Modern Automotive Technology.
For those looking to go beyond the general knowledge books, and looking for something a bit more mathematical, I have two books on my bookshelf that I’d recommend. The first, Automotive Engineering Fundamentals, gives a good general overlook of automotive engineering, and also includes good information on vehicle dynamics. The second, a personal favorite, is Engineering Fundamentals of the Internal Combustion Engine. This book gives great arithmetic insight on engines, and also includes fantastic history tidbits throughout. It’s a must have if you’re fascinated by internal combustion.