A novel that looks at the damage that war has done to a pair of individuals, or a pair of pair of individuals even, if you want to get more technical.
As usual, conflict happens when The Culture decides to meddle, or does so out of necessity, here, the nature of AI or posthuman citizens deciding to just up and leaves means one particular race is not happy with the Culture, and decides to extra an amount of revenge that will entail a large number of deaths on an Orbital.
In fact, there is a discussion about the natural of Artificial Intelligence and its behaviour, and the Culture minds themselves:
"What various Involveds including the Culture had also tried to do, often out of sheer curiosity once AI had become a settled and even routine technology, was to devise a consciousness with no flavour; one with no metalogical baggage whatsoever; what had become known as a perfect AI.
It turned out that creating such intelligences was not particularly challenging once you could build AIs in the first place. The difficulties only arose when such machines became sufficiently empowered to do whatever they wanted to do. They didn't go berserk and try to kill all about them, and they didn't relapse into some blissed-out state of machine solipsism.
What they did do at the first available opportunity was Sublime, leaving the material universe altogether and joining the many beings, communities and entire civilisations which had gone that way before. It was certainly a rule and appeared to be a law that perfect AIs always Sublime.
Most other civilisations thought this perplexing, or claimed to find it only natural, or dismissed it as mildly interesting and sufficient to prove that there was little point in wasting time and resources creating such flawless but useless sentience. The Culture, more or less alone, seemed to find the phenomenon almost a personal insult, if you could designate an entire civilisation as a person.
So a trace of some sort of bias, some element of moral or other partiality must be present in the Culture's Minds."
"To flourish, make contact, develop, expand, reach a steady state and then eventually Sublime was more or less the equivalent of the stellar Main Sequence for civilisations, though there was an equally honourable and venerable tradition for just quietly keeping on going, minding your own business (mostly) and generally sitting about feeling pleasantly invulnerable and just saturated with knowledge."
This particular race reactivates a stored military man, and partners him with another in one body. The other individual involved is a Culture Mind - but also something of a twin, as in its history early in the Idiran war it was lost and presumed destroyed during a desperate battle, a backup was made and activated, then the two met up again, with one of them dying later in the war, and the remnants merging with the survivor. This Mind is the Mind of the Orbital in question - and the time of the attack is to coincide with a celebratory concert to honor a particularly destructive long ago war event and the arrival of the light from said blowing up of stars.
Interspersed with the more serious nature of this you will find plenty of funny lines, even some indulgence by characters in a sort of word game based on the crazy Ship names that exist in The Culture.
Perhaps a bit higher than a 3.5.
3.5 out of 5