The Maker of Universes, the Gates of Creatio — Philip Jose Farmer

Robert Wolff, 66, had opened the door -- to discover not a closet, but another world. It was an Eden populated by strange and lovely creatures, and after living there a few days, Wolff actually grew young again. He saw no reason to return to Earth; the new world promised excitement and wonder. But it had perils, too, as Wolff quickly learned.