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.
Streamlining Light Truck Fuel Economy
(Details the effects an aeroshell and rear wheel skirts had on the highway fuel economy of Phil Knox's Toyota T100 pickup. Cutting drag coefficient from .44 to .33 increased highway fuel economy from 25 mpg to 27.5 mpg.) http://www.evworld.com/view.cfm?sect...le&storyid=774
Free Fuel Riding in the Wind (Details the effects aero mods had on the highway fuel economy of Phil Knox's Toyota T100 pickup. Cutting drag coefficient from .44 to .25 increased highway fuel economy from 25 mpg to 32 mpg.) http://www.evworld.com/view.cfm?sect...le&storyid=870
Aerodynamic auto designs, past and present, and their possible implications (A blog article I wrote crudely estimating the effects cutting aero drag would have on fuel economy. It will be updated sometime in the future to use a more accurate hyperboloid of best fit to include peak engine horsepower into the criteria and a sample size of over 150 cars.) http://www.evworld.com/blogs/index.c...d=87&archive=0