The Oak Above the Kings — Patricia Kennealy-Morrison

Taliesin, Chief Bard of Keltia, brother in sword and deed and marriage to Arthur the King, sets his magical Seeings into fated words. He tells how Arthur the lost prince, overthrows the evil, usurping Archdruid and at last comes to his own kingship. But it is a double throne which he must share by law and love with the one woman in all Keltia fit to share it with him. She is Gweniver, princess and priestess, his own cousin--and his wife.