Which American author wrote both The Idylls of the Queen, and the "Frostflower and Thorn" series?
The Idylls of the Queen: The Idylls of the Queen: A Tale of Queen Guenevere is a 1982 fantasy mystery novel set in the framework of the King Arthur myths written by American author Phyllis Ann Karr. Phyllis Ann Karr:  She is best known for her "Frostflower and Thorn" series and Matter of Britain works.
Phyllis Ann Karr