I don't remember a lot about the plot -- the cover was yellow and featured an illustrated brunette heroine in medieval or earlier clothing.
I think that it's a matriarchy and that the king has either been sent away or has disappeared. I think there was some sort of prophesy that he could not father the heir to the throne, so it was necessary for him to leave.
Also, I think it starts with some sort of annual festival where a handsome foreigner appears and the princess heroine makes eyes at him until her mother claims him as her consort in one of those year-and-a-day things.
Then, after the queen has delivered an heir, the princess and the now ex-consort are sent on a quest of some sort, maybe to search for the missing king????
I remember thinking at the time that I read it that it was really different from anything I'd read before, but I don't think I'd had enough knowledge of other cultures like the ancient Celtics and their less restrictive views on sex and marriage or matriarchal societies to appreciate what I was reading. Now that I'm older, more well-read, and far more open-minded, I'd like to reread it...if only I could remember it...