Subliming is mentioned in almost all of Iain M. Bank's Culture novels, but not until now has he explored the concept in any detail. It forms an important part of the plots of novels such as Look To Windward and Surface Detail, but we actually know very little about it. In the universe of the Culture, subliming is the civilizational end game; the point where a species collectively checks out of this universe and passes onto some other dimension, outside even the multiverse structure mention in Excession. Sublimed beings become so vastly powerful and complex that they care little for the trivial affairs of species still bound to this physical plane. It is a concept common to many science fiction universes, from ascension in Stargate SG1 to first ones of Babylon Five.

Banks’s latest novel, The Hydrogen Sonata, focuses on the events surrounding the subliming of the Gzilt, an pan-human civilization that nearly became a founding member of the Culture ten thousand years earlier. Mysteriously, they opted not to join at the last minute but have enjoyed close ties to the Culture ever since. Now, on the eve of their subliming, a message has been delivered which could shake Gzilt society to its foundations. Only one person can verify if this message is true: QiRia, the oldest person in the Culture who was present ten thousand years earlier when the Culture began. A Gzilt by the name of Vyr Cossont, who met QiRia twenty years ago, is dispatched to find the truth behind the message. She teams up with a band of Culture ship minds (mainly one called Mistake Not...) to, in typical Banks style, travel around some of the galaxy’s more unusual spots to find QiRia. Meanwhile she is pursued by members of the Gzilt military who are intent on keeping the message secret in case it threatens the subliming.

This book follows on from trends in the last three Culture novels: firstly the events mainly take place outside the Culture itself. Culture ships, drones and avatars are involved, but the majority of the dramatic plot focuses on events elsewhere in the galaxy. Secondly, this is his third consecutive novel with a female protagonist, and Cossont is one of his most engaging protagonists to date, flawed with youth but also strong and determined. This story also explores a few fundamental aspects of the Culture universe. It deals with the events around when the Culture was first established and gives some more insight into its history. It also explains much more about what is involved with subliming, how species and individuals sublime, what happens to what they leave behind – these are all themes of this novel.

As usual, Banks’s imagination is front and centre, and he takes the reader on a journey to many bizarre and original places. Included this time are rivers made of sand, an airship devoted entirely to pleasure, mountains of sound and sculpted moons, which are described in beautiful detail in the book.

The Hydrogen Sonata is one of Banks's stronger novels. It lacks the flaws of the weaker ones, mainly sub-plots that do not join back up to the main plot. This novel also tones down the sex and violence to a degree but still manages to keep the story extremely entertaining. This book is more thoughtful, similar to philosophical Excession rather than the explosive Consider Phlebas or the violent Surface Details. The ending is perhaps a little predictable but does have strong emotional resonance. I would certainly urge any fans of science fiction, and especially Banks's other writing, to read this novel.