Freedom's Ransom — Anne McCaffrey

Stranded on an uninhabited planet by alien overlords, Kris Bjornsen and her fellow slaves would surely have died--without the help of the exiled alien Zainal. Not only did they survive on the world they named Botany, they eventually drove their enemies away
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Now the freed slaves must struggle to find their place in a newly configured universe. In the wake of the alien devastation, Earth needs food. Botany needs technology. And Kris, Zainal, and their people need to decide what kind of world they will become.