Deathworld 2
by Harry Harrison
Jason is kidnapped by a smug enemy to prosecute for various crimes. It forces an emergency landing on a planet where the human population is declining. Technology is extremely primitive, and knowledge is divided among many small clans, each of which monopolizes what they know with jealousy. Jason uses his creativity to survive by exchanging his knowledge for protection and power in one of these clans. He designs a device that is a primitive communication device disguised; he eventually shows his girlfriend, Pyrran, where he is in a spacecraft led by Meta. (source: Wikipedia)
