Another novel by Banks set in the Culture universe, but not as “filling” as his previous novels. We get a nice look into the technicalities of Subliming, and some more intra-Mind discussion à la Excession, but the book lacks that kind of “hook” that Banks uses so often — where some long-lost secret of the protagonists comes back to haunt them.
Part of the problem that Banks has set up with the Culture is its very superiority. In Consider Phlebas, this was countered with an intra-galactic war. Use of Weapons and others concentrated on the moral choices of Contact and Special Circumstances when they meddled in less developed societies. Excession used the cop-out to introduce a threat that simply just mirrors the Culture’s force (granted, it’s the point of the novel, but still, having a deus ex machina as the center of a story is a bit … strange).
The Hydrogen Sonata introduces the Gzilt, a civilization on the same tech level as the Culture, and the conflict this time is constrained by the fact that certain elements want to keep a secret and are prepared to kill to do so, but using covert means. So while there’s ship-to-ship combat it’s constrained by this secrecy, and a hard deadline (literally!). This gives Banks some scope in keeping conflicts between two literally superhuman factions within thrilling reason.
In the end however, this feels more like an expanded novella than a novel. Recommended for Culture fans, of course!