Maker of Universes, by Philip Jose Farmer, paperback fantasy novel
“When Robert Wolff found a strange horn in an empty house, he held the key to a different universe. To blow that horn would open up a door through space-time and permit entry to a cosmos whose dimensions and laws were not those known by our starry galaxy.
For that other universe was a place of tiers, world upon world piled upon each other like the landings of a sky-piercing mountain. The one to blow that horn would ascend those steps, from creation to creation, until he would come face to face with the being whose brain-child it was. But what if that maker of universes was a madman? Or an impostor? Or a super-criminal hiding from the wrath of his own superiors?
Maker of Universes is the first volume in ‘The World of Tiers’ trilogy.”
1975 reprint of 1970 Sphere edition