Strata, Terry Pratchett
Kin Arad, a human terraformer, is approached by a mysterious man controlling supposedly impossible technology. He claims to be a crew member from the first planetary colonization attempts, but their return was supposed to be impossible. He offers her a challenge: the location of a disc-shaped world. Which, of course, should be patently impossible.
I hate to tag this as essentially “SF Discworld,” but it’s true. No, not the humor; the book does certainly draw a laugh in spots but it’s closer to the stuff found in the later fantasy Discworld novels than the things that casual Pratchett fans have come to expect from his books. Considering this book was published in 1981, I’m wondering why the later events in that series have surprised people. It’s the concepts of the impossible, joined to a subtle mockery of human behavior, that draw the comparison. I wish Pratchett had written more SF. If you enjoy other British space-set SF of the same period, this book would be a good read.