The Car Maintenance Bibles
Its a good resource for learning about the basic components of a car and how all the parts interact. Good illustrations and descriptions.
Also, go out to the local auto parts store, and get a workshop manual for your car. Its usually around $20, but its worth it as it will have the majority of information that you need to work on your car
As far as learning about modding for economy, first, get some kind of fuel economy feedback installed in your car. A vacuum gauge is cheap to start with, but a ScangaugeII or MPGuino will give you more accurate ways to quantify the data you get. Keep a fuel log. Depending on where you do the majority of your driving will determine the types of modification you may want to look into for your car. Aero mods will net you the most gains on the highway, while modifying your driving habits will net larger gains in the city.