Inversions - Iain M Banks (Book 6 in the Culture series)

In the winter palace, the King's new physician has more enemies than she realises. But then she also has more defences than even her most hardened adversaries could imagine. In another palace, far across the mountains, sits the regicidal Protector General.

His chief bodyguard also has enemies to worry about, with the threat of treachery and assassination never far away. And beneath the surface of these feudal courts - behind the spies, murders, politics and intrigues - lies an entirely di­fferent kind of threat, an entity that nobody would ever suspect.