The two I've seen referred to occasionally (by non-experts) are:
Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering ([Proceedings] / SAE)
by by
Wolf-Heinrich Hucho (Editor)
and
Road Vehicle Aerodynamic Design
by R.H. Barnard
I have Barnard's book. But from what I hear, the Hucho one is better (good luck finding it at less than $100-140).