I have to say, first off, that every single review I saw of this book online (even ones as short as a single line) gives away something you are not supposed to know until the very end, if you figure it out at all. These details that they spoil are not exactly essential to the plot, but one was spoiled for me (and I think the novel lost some of its tautness as a result) and the one that was not spoiled I was very glad wasn't spoiled because it was a minor mystery I spent the first half of the novel picking at (so again, I suspect the novel would have lost some of its appeal had I known the answer to the riddle). All of which is a very roundabout way of saying that if you want to come to the book unspoiled, avoid all online information about it like the plague.
I say right now that I will endeavor to do better than that, and give a truly spoiler-free review.
The difficulty is that without those two bits of information that so many others cavalierly spoiled, there's very little way to talk about the book. Even saying that it is a Culture novel gives you a clue to what one of the pieces of information is, but I felt that was something I could include because Banks himself gave that away. Without spoiling anymore, I will be forced to speak circuitously, which I must beg your forgiveness for. Inversions is set on a planet with a roughly Medieval level of government and medicine, and which is just beginning to experiment with gunpowder but still relies mainly on crossbows and swords. It is narrated by one of the characters, but the narrator does not tell which character he or she is, though that conceit is broken down by about the halfway point. This mysterious narrator relates two parallel tales, one of the King's physician (named Vosill) in a country called Haspidus, and one of the General Protector's bodyguard (called DeWar) in a country called Tassasen, across the mountains from Haspidus. The countries are not at war with one another, but they are uneasy about each other because the world has just suffered a planet-wide disaster which has upset all of the old systems of government.
That, then, is the set-up. The chapters alternate between the Doctor's story, which the narrator relates through her assistant Oelph, who is reporting clandestinely to another Master; and the Bodyguard's story, which the narrator relates through a third-person omniscient voice that is kept relatively confined to DeWar's perspective, but not entirely. As I mentioned above, it becomes clear who the narrator is in these stories about halfway through, but Banks handles that gracefully, not with a big reveal, but by slowly letting the mask the he or she is wearing at first slip away, almost as if unconsciously.
There is little for me to say about the two stories being told; very little happens. This novel, much more than other Banks novels I have read, is a character study, a portrait of two individuals in positions of power at a time of momentous change on this world. There is intriguing against both the Doctor and the Bodyguard, for they are foreigners to their lands and not trusted as a result; there is a touch of romance, mostly unrequited; there are surprising philosophical passages that take on greater weight as events unfold. There is a startlingly vivid hunt scene, and a botched assassination attempt, but otherwise the only action comes in a mock war (complete with catapult) that DeWar has with the General Protector's son. There is ugliness, because Banks never shies from that, and there is quite a bit of witty repartee between the Doctor and her King and between the Bodyguard and the General Protector's favorite concubine. There is also a tale of a land called Lavishia, and two cousins that lived there, that is the only real clue to the bit of information that was spoiled for me.
Ultimately, the stories end, but as with the other Culture novel I have read, the ending is pretty damned emotionally unsatisfactory. But that, too, is a stylistic choice on Banks' part, and one that I respect. They end unsatisfactorily because, unless all of humanity is obliterated, no story ever has a real ending. There will always be loose ends, people who disappear leaving only questions behind them, events that are understood imperfectly, and whose full effects still haven't been seen. It is actually a happier ending than that other Culture novel, I think; at least, the people within the story seem happy with it. The philosophical questions raised are never answered, because how could they be? They have no right answers. . . (I'm looking at you again, Prime Directive!) Instead, we are left to muddle through day by day, doing the best we can, trying to hold onto the best parts of ourselves and make good decisions with imperfect information, just as all the people (in Haspidus, in Tassasen, and even in Lavishia) in this story do.
And that's where Inversions left me, a tad frustrated (but again, I think that was deliberate), a tad philosophical, and fairly impressed. I do believe I succeeded in writing a spoiler-free review, but I'm not sure I managed to say anything at all, lol. I would definitely recommend this book, but you must accept that nothing happens, there is no real ending, and there isn't even a message to it all. That said, Inversions is still one of the strongest books I've read in a while.