But Matter, after a slow start, blew me away. It goes quite deep into the darker side of things, addressing some of the themes that have become more and more prominent in SF recently in ways that both make sense within the Culture context, and explain why they're not an issue for that context. Runaway nanotech is presented as something that was solved hundreds of thousands of years, if not longer, ago; the possibility of constant undetectable surveillance is accepted with a sort of shrug of outrage. The whole thing has a much bigger sense of history than previous Culture novels -- there are ship characters who have been around for most of the history of the Culture, and the gigayears-long history of the galaxy as a whole is evoked more successfully here than ever before, with the ancient artifacts that litter the place feeling much more present.
The book centres around a "Shellworld", Sursamen, a nested set of sixteen hyperspheres (one disappointment was that the implications of the four dimensionality here weren't really explored -- there were no brilliant bits of gonzo geometry; in fact it mainly seemed like a plot device to keep the Culture characters from using the really heavy artillery), one of thousands built way back in the early history of the galaxy by a long-departed race who sought to put a big shield all the way round it, and now -- like many others -- occupied by a variety of alien races. Among these are the humanoid Sarl, a primitive feudal society who were brought there by slightly more advanced aliens, above whom are more advanced aliens who administrate the Shellworld, who themselves are "mentored" by a waterworlder culture, the Morthanveld, who are on their way to becoming a waterworlder Culture, capital-C. (There are a lot of brand new alien species here, and they're all well done.)
The book opens with the Sarlian King's right hand man killing him after a victorious battle so that he can take control, and from there goes on to follow the king's three surviving children (his oldest son and presumptive heir having died a little while before the turn of events). The next in line to the throne, Ferbin, runs away after witnessing the murder -- his strand of the plot follows him as he leaves first his home "level", and then Sursamen entirely (a process which expands his servant's horizons more than his own, in the end), in search of the Special Circumstances agent whose advice was what enabled his father to win all those battles, or failing that his sister Djan, who was married off to/rescued by the Culture at the same time and is now an SC agent herself. Meanwhile we follow her strand as she heads back to Sursamen through the galaxy, exploring the Culture and other civilisations and her personal history, until she finally meets up with Ferbin and they head back to Sursamen to sort things out. The youngest sibling, Oramen, a bookish type who never expected to become king, suddenly finds himself Prince Regent and takes a very long time to realise the danger he's in.
Meanwhile, the aliens are really interested in one particular thing on the level of the Shellworld that the Sarl have just conquered -- an ancient city being gradually revealed by the inundation of a continental-scale waterfall (an excellent example of Banks's trademark goshwow images). It's a permanent Time Team dig, with people desperately trying to find ancient treasures before they're washed away. When the waterfall freezes due to a once-in-a-lifetime absence of any "interior stars", the opportunity to explore further presents itself, and deep in the alien city, an ancient artifact is discovered ...
The slow start for me was the Sarlian stuff -- it was all a bit high-fantasy translated to an SFnal environment, and the more we found out about how much the Sarlians knew about their place in the galaxy, the less plausible it seemed that their culture would evolve along quite such similar lines to those of Earth's past. But it is one of the points of the book and convergent evolution of both the biological and societal varieties is something you have to accept as a background assumption in Banks's SF.
A lot of Banks's standard plot elements are on hand here, but not in an overpowering way. There are plenty of big castles and such, but they don't do that thing of hulkingly occupying the psychological space of the characters, and there's a family (the three main protagonists are siblings) but not one that's really all that close. There's examination of the link between games and reality (the bravura chapter in which Ferbin finally meets the SC agent he's been seeking has a brilliant combination of this with something between Manicheanism and protest atheism to argue that we're not living in a simulation but truly are "running" in the matter of the title). But none of these gets to predominate and the second half of the narrative really hurtles towards the conclusion, with a wonderful building sense of increasing scale and doom.
Overall, it felt most to me like "What if the Culture came across an Alastair Reynolds novel in progress?" The hallmark of Reynolds's writing to me (though I haven't read his latest, which apparently is much cheerier) is the way he confronts petty humans with a vast, ancient, uncaring universe in which they're essentially irrelevant and being manipulated and lied to by everyone they meet, and this is essentially what's happening to the Sarlians throughout (one particular scene -- where Oramen is contacted by one alien faction through a comms device he doesn't know is a comms device until it starts talking to him -- was particularly Reynolds-ish in feel to me). The difference is that in Reynolds the humans might just scrape through and survive, whereas here we've got the Culture, and one SC agent, a drone and a not-quite-as-decommissioned-as-it-should-b
All in all, a real return to form. Yay.