Robert Wolff is an old man with a mysterious past. He has no memory of where he came from. But when he discovers a strange horn in an empty house, he his hurled into an amaing adventure. This horn is able to open a gateway between worlds and he's transported to a mini-universe, "The Word of Tiers". It's basically a stair shaped planet, with each step spanning thousands of miles and inhabited by all sorts of strange creatures. During his immediate stay, on a level that has nympths and intelligent gorillas, he finds himself getting younger. Then, he meets Kickaha The Trickster, who fills him in on what this place is. An there's very real threats, such as Podarge, a Harpy-like woman with the body of an eagle. Eventually he makes his way to the very top level, which is reserved for the "God" of this universe--and discovers that he may be something quite more than human.
As with a lot of Farmer's work, this is along the lines of Edgar Rice Burroughs. I like this better than the River World series but not quite as much as his Tarzan/Doc Savage pastiches. Recommended.