Matter - Iain M Banks (Book 8 in the Culture series)

The intricate structure of a Shellworld - an arti­ficial planet of spheres-within-spheres - is matched only by the machinations of its inhabitants. On the eighth sphere of Sursamen, a man witnesses the murder of his father and flees, searching for the one - maybe two - people who could clear his name. For his brother, this means a life lived under constant threat of treachery, while for their sister, Djan Seriy Anaplian, it means returning to a place she'd thought abandoned forever.

Anaplian is not who she once was. She has become an agent of the Culture's Special Circumstances section, charged with high-level interference in civilisations throughout the greater galaxy. Concealing her new identity - and her particular set of abilities - might be a dangerous strategy.

In the world to which she returns, nothing is quite as it seems; and determining the appropriate level of interference in someone else's war is never a simple matter.