The Regent's Reign — Alex McGilvery

When King Harald sets off on his pilgrimage of atonement, Marriette is made Regent, much to her dismay and that of the hidebound Council. Before long whispers of treachery surround her as her councillors, the Empire, and even the Church try to manipulate her for their own ends. The King and Queen vanish and the power struggles grow more deadly. There is more than just Marriette's life on the line if she falters.

The Regent's Reign is a follow up to The Devil Reversed, though it can be read as a stand alone book.