Some sources that have been very helpful for me:
(Mostly from the racing end of aerodynamics)
Race Car Aerodynamics: Designing for Speed, by Joseph Katz (Again, primarily racing, but a good place to understand the principles.)
Simon McBeath's book, Competition Car Aerodynamics: A Practical Handbook. I've seen McBeath quoted here before (the Advantage-cfd link above), and this is an expansion of the Racecar Engineering Aerobytes column that he writes. Again, pretty hard-core racing, but there are some great CFD pictures that help make sense of this stuff (like how a 20 mm trunklid spoiler can REDUCE drag).
Also, thanks to my local library and interlibrary loan, I've spent time with the proceeds of every SAE Aerodynamics conference from 1998 to the present. You have to do a lot of sifting, but there are some fascinating studies here. A number of them are more drag related, and may be of great use to this crowd. I have seen several studies that validate the effectiveness of engine belly pans, in particular.
I hope this is helpful!