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Dumarest had been to many worlds in his long quest for Earth, but none were to entrap him as dangerously as Zakym. A mere stop on his route, Dumarest had thought, but he had not known then of the things that made Zakym of the double-sun unique. One was the daily recurrence of the dead---the time of delusia when spectres walked and citicized the works of the living. The other was the Pact--the unwritten treaty with an unseen species that divided the world forever. In daytime it belonged to humanity; after dark it became a hostile and alien planet. Dumarest thought Zakym would be a mere stopover on his quest, but the living, the dead, and the invisible decided otherwise.
Used availability for E C Tubb's Haven of Darkness
April 1980 : UK Paperback
May 1977 : USA Mass Market Paperback
September 2011 : USA, Canada Kindle edition
September 2011 : Australia, UK Kindle edition