The basic plot is that a young woman — possibly a queen — is imprisoned when an invading army kills her husband. She is awaiting her fate — possibly death or being forced to marry this invader — when in her dungeon she is visited by a man/magician/sorcerer who says he can save her and take her away from this place. I can't remember if she ever sees this person's face, but she takes a risk and trusts him.
The sorcerer frees her by sending her through time and space to his own kingdom. When she gets there, she finds that the sorcerer who saved her is dead, but she needs to marry the king to save the kingdom from an usurper.
I think in her dreams the sorcerer might visit her to help her solve the palace intrigue. And there is a possible dream scene of two battling magicians on a lake playing chess.
The book cover, from what I can remember, was of a sorcerer — but not the ancient old bearded type — standing behind a young woman. The story is very much fantasy/medieval in location but the book makes it seem more science fiction. The story was stand alone; it was not part of a series at all.