Banks tells me that he has spent the past three months writing another Culture novel. It will be called Matter and is to be published next February. "It's a real shelf-breaker," he says enthusiastically. "It's 204,000 words long and the last 4,000 consist of appendices and glossaries. It's so complicated that even in its complexity it's complex. I'm not sure the publishers will go for the appendices, but readers will need them. It's filled with neologisms and characters who disappear for 150 pages and come back, with lots of flashbacks and -forwards. And the story involves different civilisations at different stages of technological evolution. There's even one group who have disappeared up their own fundaments into non-matter-based societies".
The working title of Banks' earlier novel The Steep Approach to Garbadale was also Matter.
A central theme of the book is the concept of galactic 'mentoring' of lesser races by more advanced one. This is mentioned in passing in other Culture novels but is formalised in Matter. In the book, the Sarl species exists on the cusp of an industrial age and is mentored by a more advanced species called the Oct, who are in turn mentored by the Nariscene who are in turn under the supervision of the Morthanveld, a species approximately of equal power and sophistication to the Culture. Flexible but relatively strong rules bind all these relationships, intended to allow less-developed species to evolve, both technologically and culturally, and to prevent them from despairing once they become aware of the powers of civilisations orders of magnitude above them.
The book follows the experiences of three members of the royal household of the Sarl, a humanoid race living on the 8th level of the Shellworld of Sursamen. The Shellworld is an ancient artificial planet consisting of fourteen nested concentric spheres internally lit by tiny thermonuclear "stars", whose layers are inhabited by various different species, guarded and mentored by progressively more advanced species, up to the level of what the Sarl call "Optimae". The Culture is considered an Optima, though Sursamen is not in their direct sphere of influence. Ferbin, the inheritor of the Sarl throne, has to flee his home level on the Shellworld after witnessing King Hausk, his father, being secretly murdered by tyl Loesp, the second-in-command. Oramen, Ferbin's studious brother, is unaware of the treachery - trusting tyl Loesp fully, and after Ferbin's disappearance at first lets tyl Loesp take on the role of regent until Prince Oramen comes of age.
The Oct, the mentoring species of the Sarl, meanwhile have been organising the takeover of the 9th level of Sursamen, using the Sarl as their pawns. It becomes increasingly clear that they are searching for something hidden in the Nameless City, a metropolis underneath eons-old layers of sediment which are currently being stripped away by the Hyeng-zhar Falls, a giant waterfall in the 9th level.
Elsewhere, Djan Seriy Anaplian, another child of King Hausk, who many years before the murder of King Hausk left Sursamen to become a member of the Culture, and of Special Circumstances, decides to return to Sursamen, originally simply to pay her respects to her dead father. On her way back, she eventually joins up with the fleeing Ferbin and his faithful (but increasingly independently-minded) servant Choubris Holse, and realises not only that her father's death was murder, but also that the Oct are planning something mysterious on Sursamen. Special Circumstances asks her to investigate.
Returning to Sursamen, they realise that they have come too late - though Oramen, warned by several botched assassinations, has now begun open struggle with tyl Loesp, neither of them could (or wanted to) stop the excavations in the Nameless City before it was too late. A member (or possibly a machine) of a lost civilisation is discovered and wakes. The Iln's sole intention is the destruction of shellworlds wherever they are found, and its real nature comes as a horrible surprise to humans and Oct both - with the Oct having thought that they were excavating one of the Involucra (the 'Veil') who had originally built the shellworlds and from whom they claim their descendance in religious faithfulness. The creature kills all present with nuclear radiation (including tyl Loesp, Oramen and several hundreds of thousands of workers excavating the Nameless City), before heading towards the core of the world, to destroy it completely using antimatter.
Djan, Ferbin, Holse and Hippinse, the avatar of the Liveware Problem, an SC-ship that brought them to the planet, head towards the core level, equipped with highly sophisticated SC-technology level combat suits, but find themselves outgunned by the Iln, who has taken over the programming of several Morthanveld combat machines emplaced in the core. Hippense and his ship die in the combat, destroying the Morthanveld machines, but the Iln alone remains too strong for the remaining compatriots. In the end, Ferbin and Djan sacrifice themselves to the 'curiosity' of the Iln, allowing the dying Djan to get close enough to the alien to detonate the antimatter reactor in her head that powers her SC-equipment.
In the epilogue, Holse, the lone survivor of the Iln encounter, rejoins his family after a long absence. He declares his intention to become a political leader of the Sarl, with the secret backing of the Culture. From Holse's survival and the continued existence of the shellworld, one must assume that Djan's sacrifice successfully foiled the Iln.