Matter — Iain M. Banks

In a world renowned even within a galaxy full of wonders, a crime within a war. For one man 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, even without knowing the full truth, it means returning to a place she'd thought abandoned forever.

But the sister is not what she once was; Djan Seriy Anaplian has changed almost beyond recognition to become an agent of the Culture's Special Circumstances section, charged with high-level interference in civilizations throughout the greater galaxy.

Concealing her new identity -- and her particular set of abilities -- might be a dangerous strategy, however. 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.