The following 2 books are great and will do much more to improve your times then anything else you can do for close to the price.
How to Autocross by Andrew Howe $24.95
Winning Autocross Techniques (Speed Secrets) by Ross Bentley and Per Schroeder $19.99
A quick Amazon search will find others, but these 2 are the newest that I'm aware of and are the 2 that I own and have read. If you only buy one, I'd recommend How to Autocross as it covers car setup and driving technique, whereas Winning Autocross Techniques is almost exclusively about driving technique, but it gets into more detail as a result.