I've been reading Fluid Dynamic Drag by S F Hoerner.
I bought the 1958 edition second-hand but the 1965 edition is still available new (see Vman 455's post below).
It is very, very interesting. It has a chapter on cars but I don't think I learned anything much there - it is largely of historic interest only. (Although he lists different top speeds of a car with different drag coefficients and the same engine power, allowing me to compare it with throttle-stop testing.)
But stuff where I think I can learn a lot includes his discussion of aircraft and airships. Why? Well, his theory and examples extend back to the 1920s and so the speeds he is talking about, and the use of piston engines, means that a lot of topics such as cooling systems, ducts, surface roughness and low drag shapes are directly applicable to cars.
The book is packed with information - as in, there are hundreds of diagrams, graphs and tables (no photos). A lot of the diagrams and graphs are very small - I will need to use a magnifying glass on some when I really want to see what is being described.
There are a lot of equations but the maths itself is almost all just multiplication/division/powers - basically, no calculus. But he loves using symbols, and because he is also using Imperial units (pounds/feet/mph) in the examples, I found a lot of the equations hard to understand. I think for some of the equations, I might just write them out in full.
For people who are really visual, a lot of the diagrams are self-explanatory eg showing drag of different ducted cooling systems.
If you buy the book secondhand, it's a hardback with good quality paper that, at least in the copy I got, has stood the test of time really well eg no yellowing. Prices seem to vary a lot.