usually it isn't hard to find authors begging to get on the radio to plug their book.
Search around for new or soon to be released books, find out who the authors publicist/publisher is and call them up. I'm sure they would jump at free air time in exchange for the chance to plug their latest tome.
Newspaper people are usually wiling to come on, especially if you are a local station, but you may need to come up with a gift certificate or something to compensate them... Can you say "trade"?