The galaxy was linked like chain mail ... It was all loops and circles and long, joined-up threads and looked like that old-fashioned stuff some old knights from the deepest, darkest shires and valleys still wore when they ventured to court, even if they rarely polished it in case it got worn away. (p. 320)
The entwined plots of Matter -- a quest for revenge, a Big Mysterious Object, a lost city full of alien tech, feudal warfare -- create a multi-layered narrative (like the multi-layered 'shellworld' that's the setting for much of the action) that showcases Banks' gift for epic fantasy as well as some impressive worldbuilding. Matter's on a par with Greek tragedy in scope and resolution, and it brings together themes from fantasy and from science fiction with innovation and wit.
Matter begins with treason and regicide, witnessed by the King's second son Ferbin. Ferbin, aided by his trusty manservant Choubris Holse, seeks vengeance: the two leave Sursamen (a shellworld of many layers, each housing an entirely different environment, with a possibly-mad WorldGod at its core) on a quest for Ferbin's long-lost sister Djan Seriy Anaplian, taken as a child and raised in the Culture. Anaplian works for Special Circumstances, and takes a professional as well as personal interest in events back home.
Meanwhile, on the Ninth Level (one down from Ferbin's homeland) an alien city is being uncovered, and the secrets it holds are of interest not only to humans but to the crablike Oct and the parasitic Aultridia, as well as other Optimae races (the Morthanveld, the Nariscene) ...
There's plenty of info-dumping but it's seldom obtrusive, and Banks tends to show then tell. The prose is generally entertaining, sparky, well-paced with some excellent writing (the winds as whining gears in the vast engine of the atmosphere (p. 410)) and some closely-observed characterisation. Marvellous world-building, too, and some amusing asides on geology (Ferbin suspects plate tectonics to be a joke dreamt up by his tutors: he's pretty clued in to shellworld astrology, where the stars -- fixed and rolling -- of the level beneath is closer than those that illuminate one's own.
What I liked most about this novel was seeing the Culture from an outsider's point of view. Anaplian is modified physically, mentally and emotionally once she becomes a part of the Culture: she marvels at and mistrusts the sheer ease of this money-less, liberal society, at drugs without side-effects and enhancements available on request, though she's unconvinced that such light [is] possible without shade (p. 169).
There's a serious underlying argument about the Culture permitting, allowing (encouraging?) the kind of feudal conflict that killed Ferbin's father, King Hausk. The stage is small but the audience great, (p. 120) Hausk was wont to say, and Ferbin gradually realises that this doesn't merely apply to the 'audience' of the populace but to a wider, greater, unseen -- and possibly hypothetical -- gallery of observers.
The more I think about Matter, the more I think that Ferbin's encounter with 'old family friend' Xide Hyrlis is the key to the whole novel. It would certainly explain the title ...
"... no matter whether we are all in a still greater game, this one here before us is at a cruder grain than that which it models ... you need to play [it] out in reality, or the most detailed simulation you have available, which is effectively the same thing."
Holse smiled sadly. "Matter, eh, sir?" (p. 348)
There's a mean trick almost at the end of the novel. I swore at Mr Banks. But despite the author's cruel and unusual sense of humour I enjoyed Matter immensely -- much more so than Look to Windward or The Algebraist.