Some lighter reading now. The other two Culture novels I've read left me with the impression that (spoiler alert) the Culture always wins. The Culture is a tremendously advanced, post-scarcity, civilisation; the most powerful in the galaxy. Excession poses the question: what happens when it encounters something that seems more powerful than it is? That it doesn't understand. For which it has no frame of reference. That might challenge it.
Tied into that is the story of the Affront, a species that enjoys cruelty and refuses to reform, and the tales of the human characters. Even more than the other books I've read, the events in this story are driven by AIs. The humans (and drones) are manipulated, commonly with care and compassion, but with little control over wider issues.
I suppose if there's a common theme running through the book it is - when should we interfere? If we think other people, societies and civilisations are doing themselves or other harm, should we intervene? The Culture is an AI-managed utopia; should we feel disconcerted by this? Or is it the crowning achievement of a civilisation to create something better than they are?
Anyway, that aside, the book is a good read. Like the ships they live on, Banks cares about his characters, and somehow he saves a number of funny bits for the end of the book.