Maybe it's only a fantasy book, but the characters seem so real that the only indication that its world is not the same as ours some centuries ago is the little magic used in it.
The main character is Tobin, a feminine child whose real identity has been hidden in order to save her life from the king who wants to suppress every feminine heir to the throne: a spell has changed the girl's appearance into a boy's. A prophecy links Tobin to the throne: she must become queen, otherwise her land will be destroyed...
The "boy" (Tobin doesn't know his real sex) is haunted by his twin brother's demon, murdered at the moment of birth, and by the dangerous presence of his mad mother. The character description is among the most spectacular I've ever read: the author tells with unbelievable richness how the boy feels. Tobin is a real child: he suffers, he hopes, he feels lonely, he fears, he loves his parents and his few friends.
Growing without any friends and only with some old servants and foul spirits trying to harm him makes Tobin wary of people and you suddenly find yourself straining to catch and help him: he is such a wonderful character that he takes hold of you and never let you go.