Roadtrip USA by Jamie Jensen
An incredible guide to the old two-lane US Highways (they were replaced by the Eisenhower Interstate System in the 50's and 60's.
The book explains about ten different routes and all of the cities/towns and things to do and see in between. Absolutely perfect for the kind of trip you're planning on taking. It won't give you access to ALL fifty states, but it's chock full of information about the lower 48.
Good luck, safe driving, and DON'T ATTEMPT TO DRIVE TO HAWAII ;)
AB