If you want to learn how to model a car from reference pictures, then check out this tutorial by Gaurav Nawani.
Gaurav covers techniques such as loading bitmaps in the 3D viewports, tracing them, mesh extrusion, normal correction and using subdivision surfaces. The tutorial is aimed at intermediate level Blender users.
You may remember Gaurav from The Making of Ambassador Summers.
The tutorial has been on his site for more than a year, but it's still excellent learning material and we definitely want to have this one in our library.
Youl'll find it here.
Update: Gaurav's site traffic maxxed out. For the time being, I've put a local copy on this site and routed it through the Coral Cache. The links may display slowly, but they work: