"The Maker of Universes" by Philip Jose Farmer (1965)
This is the first book in Farmer's World of Tiers series. I reviewed the second book, The Gates of Creation, over two years ago.
The author, Philip Jose Farmer, is most famous for his Riverworld series, which bears many similarities to his World of Tiers. I have yet to read any of the Riverworld books, but I suppose I will eventually.
In The Maker of Universes, the aging Robert Wolff finds a portal into another dimension. After entering into this other world, he finds himself growing younger, and also crosses paths with a distant figure referred to as The Lord of the Tiers. To find this mysterious person, he has to advance through the World of Tiers, a ziggurat-shaped universe in which worlds are layered one upon the other, with the Lord of Tiers occupying the highest level.
It's a lot like Edgar Rice Burroughs meets Space Trilogy-era C.S. Lewis, with a lot of obscure vocabulary thrown in for added effect. Wolff advances very quickly from desperate senior citizen to mythic hero, and the type of world that Farmer builds adds a luster that many other pulp adventures seem to lack.
It is, however, written in an amateurish manner, and some of the transitions between "scenes" are awkwardly handled. The book's conclusion is somewhat arbitrary, and it feels like Farmer was at a loss for how to end it. Some of the word usage is also questionable ("twilightly"?), but having already read the sequel, I can assert that these flaws were addressed/corrected in later books.
It's not great by any stretch of the imagination, but it is consistent. It's also the gateway (excuse the pun) to other, better books in the series, and for this reason I would recommend it.