I'm a little biased on this one: it's about the fourth time I've read it, and I think it's one of the best SF books ever written. It opens with the line "The bureaucrat fell from the sky."
The basic plot sees "the bureaucrat" (whose name is never given) sent to the planet Miranda, where the once-every-200-years tide is about to come in. It will cause Ocean to rise by tens of meters, possibly hundreds (that's never explained ... nor do I think the "science" of the catastrophically melting polar ice caps that power this phenomenon actually make any sense ... but it's still cool). He's been sent, with almost zero authority or power, to track down an off-world educated local called Gregorian who has been running TV ads claiming he has access to off-world (and possibly non-existent) technology that should never have reached Miranda. He is also what the Mirandans call a Magician.
The book is surreal, full of strange scenes and weird digressions, stories within stories. And yet it paints what I found to be a very believable future - including a situation where those in space dictate what technologies those on the surface can use, with the intent that the failure of a technology doesn't lead to the collapse of planetary civilization. Which inevitably leads to frustration as people planetside are aware of what's withheld.
Swanwick has always had a way with delayed understanding - he gives you parts of a puzzle and leaves you staring at them for half the book or longer before he hands you the remaining pieces. I'm not talking about the major plot driver, that's expected, but other details as well. Closely tied to this is his habit of off-hand revelations: small pieces of information that completely change your thinking (or the character's, or both) about what's come before. He doesn't play these things up, it's just there. So pay attention. The ending is equally surprising and brilliant.