This complex work, amazingly, was Banks' first work of science fiction. It introduces his futuristic, galaxy-spanning human society called the 'Culture' that features in a number of his other, later works (such as Inversions, which was the first of his books that I read, and which I now see in a new light). It also follows the adventures of a human soldier-of-fortune during an interspecies war, when an alien race has invaded the Culture's sphere of hegemony.
The soldier is capable of 'mimicking' other humans, thanks to the extensive genetic modification of his people (a human sub-species). As with a number of his people, he is working for the aliens out of spite for the Culture.
It's an entertaining read, but more for the universe it introduces than the story it tells. It's almost like Lord of the Rings in the grandeur of its setting in that regard (though with fewer new languages).
The main character is susceptible to a trend that happened in the pre-Gibson era, where the hero is up ultra-capable, achieving far more than is believable. And the main character's motivation is left somewhat unexplained, which is a pet peeve of mine (that, or I was too dim to see it, or just under-estimated the character's rage at being of this sub-species that seemed to have been engineered by the Culture, I'm not sure).
In any event, I intend to dive into the rest of the related works, and I recommend this one.