Knight or Knave by Andre Norton

Times are changing in Rendelsham. The old King is dead, and the foolish Prince Florian has assumed the throne. Florian's mother, Queen Ysa of the House of Oak, still controls the land from behind the scences, but her job grows more difficult every day. Her unworthy, headstrong son is harder to control than her husband was, and she must spend more time than ever masking her own movements. Her husband's illegitimate daughter Ashen, heir to the nearly dead House of Ash, still causes trouble by her very existence, and must never be given an opening to the throne. The barbarian Sea-Rover clan presents problems from the edge of the Bog, Ysa's newest magical ally has been exposed as a traitor, and nothing is going as Ysa had planned.

And still the unknown yet encroaching threat from the North continues to grow.

Through births and deaths, marriages and duels, love and betrayal, magic and force, the four Houses of Rendelsham can only survive by the strength of their unity--but is unity possible in such a court of intrigue as this one?

This book is part of the "The Cycle of Oak, Yew, Ash, and Rowan" series.
Here are some other books from this series: