I took two Iain M. Banks books away on holiday, this was the other one. Look to Windward is also set in his Culture universe, this time centring on some visitors to a Culture Orbital. An Orbital is a massive artificial habitat orbiting a star inhabited by tens of billions of people (human, alien, AI), all run by a single AI. As the story opens the Orbital is gearing up for ceremonies to mark the appearance of light from two supernovas that are 800 light years away. They weren't natural, they were caused by a weapon wielded during a war that the Culture was involved in - and the AI that runs the Orbital, called Hub, was a warship during that war. While the weapon wasn't used by the Culture they feel responsible & guilty for their involvement in a war that lead to such terrible acts & terrible loss of life. Hence the marking of the light reaching the Orbital. One of the non-Culture protagonists is Ziller, a composer in self-imposed exile from Chel, who is composing a new piece for the occasion. Quillan, another Chelgrian has recently arrived on the Orbital, ostensibly with the mission of meeting with Ziller & persuading him to return home. But all is not what it seems here & we find out (along with Quilan) via flashback spaced out through the story. Quilan is also a veteran of war - a war caused by the Culture's meddling in his civilisation's politics, for which they now feel terribly guilty.
It's been ages since I've read these books, and in my memory the Culture was always very much The Good Guys. But it actually seems more ambiguous than that. I mean, it'd be pretty cool to live in the Culture - it's a true utopia, and post-scarcity one too. A Culture citizen seems pretty much to be able to do what they want to do and live how they want to live. However the overall civilisation is definitely prone to hubris when it comes to dealing with other civilisations. They (or at least Special Circumstances) meddle, and meddle "knowing" that they Know Best. And when it goes wrong, they're oh so terribly sorry but they don't seem to learn from it - 800 years on from a war that culminated in two supernovae they're still meddling in others' politics before they know enough to do so.
Culture AI are also definitely not bounded by human feelings about unnecessary brutality when they are "off the leash" and undertaking reprisals. Both the drone Skaffen-Amtiskaw in Use of Weapons (post) and the unnamed weapon who appears in two vignettes in this book seem glad of the opportunity to cause suffering when they've got an excuse. Both scenes are unsettling because of the gleefulness of the AIs in question.
I had a couple of quibbles with the structure/pacing of the story. It's obvious from the beginning that both Quilan and Hub are veterans of war, but the other parallels between them don't appear till late on in the book just before you need to know about them for the ending to make sense. It would've been nice to have that seeded in the story earlier, but maybe I just missed some clues. There was also a sub-plot with an off-world Culture citizen who discovers the true plan for Quilan and is trying to get back to warn them. And it just didn't really seem to go anywhere in the end. I mean, don't get me wrong, it was neat - particularly as this Culture citizen was studying another alien ecosystem where the aliens were truly alien rather than just differently shaped. But I'm not sure what it brought to the overall story.
It did tie in thematically by the end, though - memory & identity were a part of the ending of that thread as they were for the other threads. So far all three Banks books I've read have had something about identity, and concealment of parts of oneself - either internally or externally imposed. I'll be looking out for it in the next ones.