Excellent Culture yarn that now feels more like a swan song than I think Banks could have intended, because it deals mostly with what happens when a civilisation feels it can't progress any more. Lots of intersecting subplots hinging around who knows what and the limits to even the god-like Culture Ships' ability to cross space and time... subplots that by the end get woven together coherently.

There's also a strong theme here about whether knowing the truth about things matters. If I didn't know it had been written a few years ago, I could easily have taken it as deliberate commentary on today's society and politics, but I suppose that's just a mark of great fiction: however much it's set in escapist sci-fi utopia it's of course also about people and how people interact.