Thousands of years ago, the Culture encountered an Outside Context Problem. A perfectly black sphere materialised out of nowhere next to a trillion-year-old sun from another universe. It did nothing and vanished. Now it has returned, and both the Culture and a hostile alien race known as the Affront are desperate to uncover its secrets.
Excession was originally published in 1996 and is the fourth novel in Iain M. Banks's Culture series. As with all of the Culture books, it is a stand-alone novel sharing only the same background and setting, with minimal references to the events of other books and no characters crossing over.
A plot summary of the novel makes it sound like Banks's version of a 'Big Dumb Object' book, a novel where the characters are presented with an enigmatic alien entity and have to deal with it (similar to Rendezvous with Rama or Ringworld). However, this isn't really what Excession is about. Instead, the novel operates on several different levels and uses the titular artifact as a catalyst for a more thorough exploration of the Culture and its goals, as well as a more human story about relationships and change.
Excession is the first book in the series to explore the Minds, the (mostly) benevolent hyper-advanced AIs which effectively run and rule the Culture (as both spacecraft and the hubs of the immense Orbital habitats). Previous novels had portrayed the Minds as god-like entities whose vast powers allowed the various biological species of the Culture to live peaceful lives of post-scarcity freedom. Aside from their whimsical sense of humour and tendency towards ludicrous names, the Minds had not been fleshed out much in the previous novels. Here they are front and centre as several groups of Minds attempt to deal with the Outside Context Problem, or Excession, and find themselves working at cross-purposes. One group of Minds appears to be involved in a conspiracy related to the object's previous appearance, whilst another is trying to flush them out. Another Mind appears to be operating on its own, enigmatic agenda. There are also Minds belonging to the Elench, an alien race closely aligned with the Culture but who may have different goals in mind in relation to this matter.
Banks depicts communications between the Minds as something between a telegram and an email, complete with hyperlink-like codes (in which can be found some amusing in-jokes). Following these conversations is sometimes hard work (especially remembering which ship belongs to which faction), but worth it as within them can be found much of the more subtle plotting of the novel.
The stuff with the Minds and with the alien Affront (think of the Hanar from Mass Effect but with the attitude and disposition of Klingons) is all great and somewhat comic in tone, but the book also has a serious side. Several human characters are dragged into the situation as well, and it turns out two of them have a past, tragic connection that one of the Minds is keen to exploit. It's rather bemusing that Banks drops in a terribly human drama into the middle of this massive, gonzoid space opera, but the juxtaposition is highly effective, giving heart to a story that otherwise could drown in its own epicness.
Excession (****½) is, as is normal with (early) Banks, well-written and engaging, mixing well-drawn characters (be they human, psychopathic floating jellyfish or Mind) with big SF concepts. The book's only downside is a somewhat anti-climactic (though rather clever) ending. The novel is available now in the UK and USA.