Iain Banks' Culture series doesn't lend itself to easy summations. Entire essays have been written on the world he set up, so I won't go into much detail.
Essentially, Banks has created a liberal utopia on a galactic scale. Given a limitless supply of easy energy and near-omnipotent manufacturing technology, the citizens of the Culture want for nothing material. Tedious administration is done by artificial intelligence, leaving normal folks to live their lives as they please. In such a society, property is an archaic concept, something that is even reflected in their language. With scarcity removed from the equation, the only real crime is coercion.
Still, every society hits a wall eventually. In Banks' world, self-destruction or a collapse into barbarism is unlikely, and the end point for civilizations is simple ennui. What do you do when you just feel like you're going through the motions, with nothing left to contribute?
Well, there's always suicide.
In Banks' world, there's the option of Subliming, in which a society (or at least a significant portion of it) can simply leave the universe and ascend to a higher plain of being. The concept has been mentioned in prior books, but this is the one in which he goes into detail.
The title is a reference to a piece of music designed to be unplayable, written for an instrument not invented at the time (the Antagonistic Undecagonstring), and unlistenable in any case. Of course, that doesn't stop people from trying, and Vyr Cossont, the book's protagonist, is one of those.
Cossont is a Gzilt, from a race which predates the Culture. They're preparing to Sublime, and her life's work is to play the piece in its entirety before that happens. Typical of Banks' literature, she's had two extra arms added to her body to make this feasible.
Of course, if everything went as planned, this would be a short book. Her people are leaving the mortal fold in a few days, but a messenger approaches to inform them that their founding holy book (which is unique in that all its predictions have been true) is an elaborate sham, and that the whole thing was a sociological experiment in really bad taste on the part of an ancient race. This could endanger public support for Subliming, so a faction within the Gzilt government has the messenger killed.
What follows is the sort of chase-and-intrigue tale Banks has done so well before. The only problem is that he's done it before.
Powerful starships bickering amongst themselves like slightly-more-competent Olympians? Check. A chase for a McGuffin with all the answers? Check. Alien court intrigue? Check.
So, yeah, it should be tedious. To Banks' credit, it rarely is. The starships are inhabited by the Minds (capital "M"), the hyperintelligent AI's that run the Culture. In a benevolent anarchy, they're basically the government. They're aware of the need to manipulate events with great care and subtlety, and their conversations are as poignant as they are sometimes hilarious.
There's an interesting contrast between the Gzilt and the Culture, who they declined to join back at the beginning. Though not militant, the Gzilt society is built on a system of military stratification, in which almost everyone has a rank, even if they're not serving. The trappings of military life give their society its structure.
On the other hand, the Culture lives a life of enlightened hedonism. War should be against their very nature, but they've had to engage in it on a huge scale, and they're darned good at it when they need to be. Nonetheless, they find it distasteful, and with the exception of a few warships kept in reserve, they do their best to avoid any mention of it.
In the beginning of the book, it's made obvious that the two civilizations regard each other as friendly equals. The Gzilt have decided to pack up and leave, and the Culture's there to say their goodbyes and make sure everything goes smoothly. This includes making the appropriate gestures, as well as keeping the looming lower-level scavenger civilizations civil.
As the book progresses, we're treated to the usual space-opera bordering on soap-opera Banks excels in. Things come to a head, then…well, it fizzles out a bit. All the things that were going to happen at the beginning of the book happen anyway.
A bit of a rip, wouldn't you say? Well, perhaps not.
Inertia and inevitability seem to be the themes here. Cossont and the Minds find the truth they're seeking, even if it means bothering the heck out a 15,000-year-old man who just wants to be left alone and immerse himself in the plantary equivalent of a My Bloody Valentine song. Aside from breaking up an epic party and getting a bunch of folks killed, nothing changes. Telling the truth to the Gzilt might screw things up for them, or it might not, but is that really the Culture's call to make?
In Look to Windward, the Culture's attempt at benign meddling led to the breakdown of a race's caste system and a civil war that ravaged their planet. They had the best intentions, but the result was a real bummer. So, in a post-scarcity society in which coercion is the only real crime, where does an entity like the Culture cross the line from altruism to manipulation?
So, yes, the book drags a bit in spots, and when I first closed the cover, I felt like Banks was treading water. Then things started to sink in. At its heart, this is a brooding rumination on how death becomes a choice, and how a society deals with that.
It's absolutely worth reading, but if you're new to Banks, it is not the place to start. If you want to go to the beginning, Consider Phlebas is the first book in chronological order and well worth reading. The Player of Games is a great standalone entry. Look to Windward, is one of sci-fi's great tragic tales, on par with Childhood's End or Thunder and Roses. Excession is a fun read, though not as essential.
Use of Weapons is one of my favorites in the series, though the narrative style can be difficult (one thread runs backward in time, while the other runs forward. It works). Surface Detail, while essential, is one saved for after you've gotten well immersed in the universe.