The Magic Labyrinth: The Fourth Book of the Riverworld Series

Paperback. English. Publisher: Grafton Books. 1989. Used. 

Now Burton and Clemens, who have traveled for more than thirty years on two great ships, are about to reach the end of the River. But there is a religion, The Church of the Second Chance, that has grown up along the River and its adherents, possibly inspired by aliens, are determined to destroy the riverboats. A coming battle may destroy Burton and Clemens, but even if they survive, how can they penetrate the alien tower of the Ethicals, who created this astonishing world? What can humans do against a race capable of creating a world and resurrecting the entire human race on it?