Book 51: "Inversions" by Iain M. Banks

If you hadn't read any of the other Culture novels, you wouldn't understand what was happening in "Inversions" at all, as it seems to be a fantasy novel about the bodyguard of the Protector of a land that seems to be at a similar level to 16th/17th century Europe, and the female doctor who attends the King of a nearby country. The doctor's story is told by her assistant in the form of reports to his master who is getting him to spy on her, while the bodyguard's tale . . .

But when you know the ways of the Culture, it is clear that the unwitting narrators of the tales are actually describing a Special Circumstances mission, and that agents have been sent down to influence the behaviour of two of the more moderate and forward thinking rulers, and prevent them from coming to harm while they carry out their reforms. ( spoilers )

Loved it. It's a million times better than "Excession".