Freedom's Choice by Anne McCaffrey
They had called the planet Botany, after the old penal colony on earth. For that is what they were - prisoners and dissidents from other worlds whom the hated Catteni had banished to an empty planet - or what they thought was an empty planet. Kris Bjornsen and her fellow slaves had survived very well - and one reason was that amongst their number was Zainal, a high ranking Catteni who was as trapped on Botany as they were. Zainal knew the Catteni ways and the Catteni technology, and he had plans for fighting back. For, as he explained, the Catteni too were victims - subject to the mighty and terrifying Eosi race who used the Catteni as a galactic police force - and also used them in more grisly and horrifying ways. And over Zainal's daring and secret plans, over the surveying expeditions and the exploration of the strange hidden valleys, hung a further mystery - to whom did Botany really belong? Who had created the giant grain sheds - the mammoth machinery that tilled the great fields? The new inhabitants of Botany called them the 'Farmers' - and waited for the day they would come to harvest their crops. And when that happened, the refugees were awed into silence - for the Farmers were greater than anything the universe had ever seen.