Diana Santee, Spaceways Agent #1: Mind Guest by Sharon Green

Diana Santee woke to find herself helpless on a starcraft bound for nowhere with all its controls destroyed. Beyond all known space she was saved by the secret outpost of an undiscovered star federation. They needed a girl like her for a desperate mission on a medieval world they were secretly probing. She fit the bill; she volunteered in gratitude. But when she became the unwanted mind guest of a terrified virgin princess, it went beyond anything her space agent training had prepared her for…