Excession

Excession is a science fiction novel by Iain M. Banks and the fourth published to feature The Culture. It is a large-scale space opera with a complex plot following many characters across large volumes of space.

Warning: Wikipedia contains spoilers

The book details the Culture's response to an "Outside Context Problem", the Excession of the title (essentially a Big Dumb Object, a perfect blackbody sphere that appears mysteriously on the edges of Culture space, is Mysterious and Ineffable for a while, and then disappears). One of the main plot elements is the relationship between the Culture and a civilization appropriately named the Affront, that tends to view all other sentient beings as tools for their amusement. The book more than any of the other Culture novels focuses on the Culture's sentient ships as protagonists.