Matter (Culture Series #7) by Iain M. Banks

A novel of dazzling wit and serious purpose. An extraordinary feat of storytelling and breathtaking invention on a grand scale, it is a tour de force from a writer who has turned science fiction on its head.

"Unexpectedly savage, emotionally powerful, and impossible to forget." —The Times

In a world renowned even 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, even without knowing the full truth, it means returning to a place she'd thought abandoned forever.

Only 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.

The Culture Series
Consider Phlebas
The Player of Games
Use of Weapons
The State of the Art
Excession
Inversions
Look to Windward
Matter
Surface Detail
The Hydrogen Sonata