FOR TRUCKERS, VACATIONERS OR JUST PLAIN ROAD WARRIORS, FINDING THE right radio station has always been a matter of guesswork. In other words: Don't they listen to real music around here? Relax. After seven years of research and travel along some 20,000 miles of America's interstates, Paul Rocheleau has compiled this book, a countrywide, two-volume guide to AM and FM stations (Berkley, paper, $8.95 each). It lists 20 radio formats in 68 cities, giving call letters and signal strengths. Eastern and western editions make it easy to find jazz in Nashville, a Spanish language station in Duluth or surf hits in the Big Apple. Just don't try to read all this small print while you drive.