One such message is a letter-in-a-bottle left by the Zihdren, themselves long Sublimed. The Gzilt warship that receives it promptly blows the bottle to smithereens. Thus begins a political incident.
Banks's trick in SF -- which he does very very well -- is to convey the heft of civilization at the galactic and millennial scale, its concerns and views, by making it all immediate and personal and vernacular. Why not? A Culture Mind can recall everything that happened on a planet in the past century like you recall what you ordered for dinner -- and so a bunch of Minds working out what to do with a planet will sound like you and your friends splitting the check. The tone fits the scale.
Since it's Banks, the tone is also snark-tastically funny; or (when appropriate, in a different civilizational point of view) pompous and absurd. He's got control of it, is what I mean.
Culture books have trouble with plot -- since there are usually several might-as-well-be omnipotent beings hovering around, ready to pull off whatever ex machina is required. I think Banks got that figured out several books ago. This one is plot-shaped, as a spy thriller, with starships flying hither and yon searching for clues or trying to blow them up before they're found. But this is superficial; it's just to keep the momentum up.
The book is... history, I guess. History at many levels: a human (Gzilt) and her life, a military power-play, a political intrigue, a Culture Mind working group. (Seriously, the whole book is worth it just for the Minds bitching at each other.) The bits of history fall together, collide, and then fall out with gaps and holes and blatantly unpulled threads. If you try to read it as a spy thriller, it will probably be an unsatisfying disaster. What is it? Oh, right: a character novel where the characters may be humans, cities, governments, or civilizations. Each a personality, in presentation if not in literal Mind.
(Yes, my model is a little askew for the Gzilt, who are Culture-equivalent but eschew building Minds for aesthetic reasons. Not entirely clear how they make that work, by the way -- doesn't it undermine the implicit rationale of the Culture? The book doesn't go into this.)
Anyway -- thriller or history or character portrayal, it works great as a Culture novel. I loved it. There are probably classical literary models that fit better, but I'll stick with that.