Plot summary from: http://edelweiss-ass...3_WATERMRKD.pdf
An ancient people, they helped set up the Culture ten thousand years earlier and were very nearly one of its founding societies, deciding not to join only at the last moment. Now they’ve made the collective decision to follow the well-trodden path of millions of other civilizations; they are going to Sublime, elevating themselves to a new and almost infinitely more rich and complex existence.
Amidst preparations though, the Regimental High Command is destroyed and Cossont is blamed. Wanted dead — not alive. Now, aided only by an ancient, reconditioned android and a suspicious Culture avatar, Cossont must complete her last mission given to her by the High Command — find the oldest person in the Culture, a man over nine thousand years old, who might have some idea what really happened all that time ago. Cossont must discover the truth before she’s exiled from her people and her civilization forever — or just plain killed.
Learning more about subliming could be good or bad. As is, I like that it remains so mysterious to even the Culture's top minds. I doubt this will change, but the book's subject matter does seem to demand that we find out something more about it.
I've heard some people express the view that Banks should kill off the series, but I thought that Surface Detail was his best since Excession. I liked Matter but it had a weak ending and just wasn't up to his previous standards.