This is the second Culture novel I've read, the other being "Consider Phlebas".
I was a bit disappointed by the previous book's outside view of the Culture itself, which I had seen described in advance and was curious about. This novel similarly has primarily an outsider's perspective, but it still manages to give a better glimpse into what life in the Culture is like, with several interludes of luxurious post-scarcity hedonism, and an apparent near transcendence of aging and death.
The plot concerns the moral grey areas and hypocrisies of the Culture's interactions with other societies. They abhor violence themselves when confronted with it, but are not above employing a man like Zakalwe to wage war on their behalf, or abandon their allies on a whim in the wars they choose to fight. Yet they do so in order to prevent larger scale wars, and retard the growth of fascist movements that don't consider everybody in the Culture, and many outside it, to even be people.
I found the structure of the novel a bit confusing, especially initially. Alternating chapters follow two strands of Zakalwe's life, with one moving forward and one backward in time. They come to a head in a pair of chapters, and an epilogue, that reveal exactly the type of monster that the Culture has partnered with. The payoff is worth the confusion.