Prison of Night — E.C. Tubb

DUMAREST
Urgent message from Central Intelligence to local Cyber agent: "You will proceed to the planet Zakym with the utmost dispatch. Dumarest is not to be killed or his intelligence placed in danger. This is of utmost priority. Once found he is to be removed from the planet immediately.

"Zakym is approaching a critical state as regards the stability of its present culture. Find Dumarest and move him before he becomes embroiled in a war!"

But Earl Dumarest, seeker of Lost Terra, was not ready to leave Zakym, world of the night rulers and the day people. He had a romance to defend, he had a mystery to be solved, and he had his next step to the mysterious Sol system to be determined.