At last weekend's WDCR event I got a chance to do a course walk with Sam Strano and chat with Brian Garfield for ~10 minutes afterwards. As I already read Secrets of Solo Racing, Brian recommended an unusual book for me to read next -
A Twist of the Wrist by Keith Code
While it's about motorcycle road racing, most of the principles apply to autocross. I just ordered a copy from B&N in the used section for $12.37.