Matter - Iain M. Banks

In a world renowned within a galaxy full of wonders, a crime within a war. For one brother it means a desperate flight, and a search for the one - maybe two - people who could clear his name. For his brother it means a life lived under constant threat of treachery and murder. And for their sister, it means returning to a place she'd thought abandoned forever.

Only the sister is not what she once was; Djan Seriy Anaplian 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 Anaplian returns, nothing is quite as it seems; and determining the appropriate level of interference in someone else's war is never a simple matter.