Excession, first published in 1996, is Scottish writer Iain M. Banks' fourth science fiction novel to feature the Culture. It concerns the response of the Culture and other interstellar societies to an unprecedented alien artifact, the Excession of the title.
The book is largely about the response of the Culture's Minds (AIs with enormous intellectual capabilities and distinctive personalities) to the Excession itself, a black swan event, and the way in which another society, whose systematic brutality horrifies the Culture, tries to use the Excession to increase its power. As in Banks' other Culture novels the main themes are the moral dilemmas which confront a hyperpower and how biological characters find ways to give their lives meaning in a post-scarcity society which is presided over by benign super-intelligent machines. The book features a large collection of Culture ship names, some of which give subtle clues about the roles these ships' Minds play in the story. In terms of style, the book is also notable for the way in which many important conversations between Minds resemble email messages complete with headers.
The Excession of the title is a perfect black-body sphere that appears mysteriously on the edge of Culture space, appears to be older than the Universe itself and resists the attempts of the Culture and technologically equivalent societies (notably the Zetetic Elench) to probe it. The Excession is what the Culture's social scientists describe as an Outside Context Problem, one which a society cannot foresee and is often fatal.
A group of stereotypical bureaucrats tries to manage the Culture's response to the Excession but is brushed aside by the Interesting Times Gang, an informal group of Minds some of who are veterans of the Idiran-Culture War, by far the most serious challenge the Culture had previously faced.
Meanwhile a rapidly-expanding race, which the Culture calls the Affront (because of its systematic sadism towards subject species, its own females and junior males, despite being relatively advanced), tries to exploit the Excession by infiltrating a store of mothballed Culture warships and using them to claim control of the mysterious object.