This is my favorite work of German science fiction, happy to see it mentioned here. Warning, the Wikipedia page contains major spoilers (as usual).
The page mentions "Quest" as a prequel, which it technically is, though really it's better thought of as a different story in the same universe. The more recently released "Eines Menschen Flügel" (roughly: Wings of a Human) of the author is also set in that universe.
This is a bit of an odd book. I remember disliking it when I read it, feeling it was a bit of a trick story. But it's stayed with me in a way few stories have.
With hindsight, I would say it's a fairytale in science fiction clothing. You have to read it with your brain in a mythic, analogical mode, rather than a literal and critical one.
And it's only 300 odd pages, so you might as well give it a go.
The page mentions "Quest" as a prequel, which it technically is, though really it's better thought of as a different story in the same universe. The more recently released "Eines Menschen Flügel" (roughly: Wings of a Human) of the author is also set in that universe.