Reddit reviews Automotive Technology: Principles, Diagnosis, and Service (5th Edition)

I suggest a 1996-2006 vehicle, perhaps a Honda or Toyota, with 200k+ miles on it and existing electrical and engine performance problems. Nothing severe enough that it doesn't run, but so that you have plenty of work to do. Learn the basics (fluid/filter maintenance etc) first. Then learn all the electrical you can. You're gonna need that for everything else.

As for knowledge, I don't remember the name of the book but I remember one of the authors was named Haldermann. (edit: http://www.amazon.com/Automotive-Technology-Principles-Diagnosis-Service/dp/0133994619) It's a text book, probably titled something like Principles of Automotive Technology Version (whatever). It should cost you around $150 and will tell you practically everything you need to know. Actual work and actual technicians can tell you a lot more but that's a very good start. I have this and reference it often.

If you want to take a class, take an Automotive Technology Electrical class and get a quarter's worth of studying electrical and hopefully troubleshooting electrical. If you have any questions feel free to ask.

If you really want to learn about cars and how they work here is probably the best single book you can get, although it is used in technical school and community colleges it is very detailed. Also the book is very expensive but if your serious about working on cars this information is important.

Automotive Technology: Principles, Diagnosis, and Service (5th Edition) https://www.amazon.com/dp/0133994619/ref=cm_sw_r_cp_api_MuAEyb881KPHW

You can find a used copy of an earlier edition of this book for cheap on amazon. Great introductory material: Automotive Technology: Principles, Diagnosis, and Service (5th Edition) https://www.amazon.com/dp/0133994619/ref=cm_sw_r_cp_api_4-0aAb64GYAWN