The World of Tiers series by Philip Jose Farmer, which I remember rather fondly from decades ago, is on sale at Amazon: $2.99 for the first three books.
His past a mystery and his present unbearably mundane, Robert Wolff is simply trying to buy a new house in Arizona when he stumbles upon a secret doorway through space and time and enters the World of Tiers. Made up of ascending levels of jungles, plains, medieval cities, and, at the top, a Garden of Eden, and populated by fantastical creatures, from nymphs and centaurs to merpeople and strange amalgams nonexistent on Earth, it’s beyond anything Wolff could have imagined in his previous humdrum existence. And when his youth is restored in the bargain, it seems he’s truly found paradise.
But there are dark forces in this new world, and Wolff is plunged into an epic quest up through the tiers, accompanied by Paul Janus Finnegan, another earthling, now known as Kickaha. Wolff’s journey to find Jadawin, the Lord of this world, will lead to answers about his own identity—and determine his fate.
Very pulp fiction-y; I have to admit that looking back on it now I’d class it as Fantasy Adventure For Boys.
Fun, though. I definitely enjoyed it back then and might like to re-read now.