It Could Be Anything by Keith Laumer, Science Fiction, Adventure, Fantasy

THE WEIRD WORLD BEYOND
Brett had to get out of Casperton.
It was a small town, Casperton, and although Brett loved his Aunt Haicey and would miss the lovely Pretty-Lee, his girl-friend, he wanted to see the wide, wide world beyond.
The ocean! The mountains! Big cities! Brett had only seen pictures of them before he got on the train that fateful day . . .
But then the train had stopped. Brett discovered there was no engine and the tracks ahead. . . . stopped!
He walked onward and found a city. . . . but it was a city was full of "golems." A true human being Brett met, Dhuva -- was taken underground by the fearsome secret masters of the city -- the Gel.
Brett had to save Dhuva and unlock the secret of this city . . . and of "reality" itself.