So Jo Walton’s Thessaly series is definitely one-of-a-kind. I feel very confident in saying there’s not a whole lot like this out there, especially by the time you get to the third book.
So the premise (which happens quickly enough into the book that I don’t think it counts as a spoiler) is that Athena and Apollo—the literal Greek gods—decide to set up a city by the plan in Plato’s Republic. They gather up a bunch of interested philosophers and scholars from throughout time, grab a handful of robots from the future, and set about building a city on an island that lies outside of recorded history. After a while, they buy up a bunch of ten-year-old children from slavers in various times and places, and then it’s off to the society-building races.
The first book follows the evolution of the city and the people within it as they try (or, occasionally, not) to live within Plato’s dictates. There’s a degree to which its arc is predictable—hands up if you really think that the Republic would be workable with actual humans living in it—but it also goes in unexpected directions.
Book two takes things into even less expected directions. And book three, which is straight up science fiction, is by far the strangest of the lot, and it’s a miracle that it even works as a novel.
But somehow it does. Somehow, the whole series hangs together, when all logic and common sense says that it won’t. It’s very distinctively Waltonian—if you had stripped off all authorial identifiers, I wouldn’t have gotten far in before I suspected the author, and by the third book I would have been iron-clad certain of it—and I can’t imagine that every reader would like it. But if you’ve liked her other stuff (particularly the also-unusual Lifelode, which this isn’t exactly like… but which may be more like this than anything else I can think of), you’ll probably like this.