If you are referring to working on automobiles, I highly recommend the Haynes book found at auto zone (probably most all parts stores will carry them). It only costs about 25 dollars (been a while since I bought one). But they will walk you through everything. YouTube is also a good site for how too, if you know specifically what needs to be changed. Good luck and God bless. :)
If you are looking to learn how an automobile engine works, then there are various online resources for that. This information is available on www.howacarworks.com/, including illustrations and repair tips. A qualified mechanic can also help you out on the same.