The Bleak and Empty Sea (The Merlin Mysteries #3)

Merlin the magician investigates foul play in the deaths of Tristram and Isolde in this Arthurian mystery novel by the author of The Knight of the Cart.
 
A disturbing report has just come to Camelot: Sir Tristram has died after a skirmish in Brittany, and his longtime mistress, La Belle Isolde, Queen of Cornwall, subsequently died of a broken heart. Hearing this news, Queen Guinevere and her trusted lady Rosemounde suspect that there is more to the story than they are being told. Now, it is up to Merlin and his faithful assistant, Gildas of Cornwall, to find the truth behind these untimely deaths. 
 
Taking a ship to Brittany to investigate, Merlin and Gildas are met with an unexpected reception. Tristram’s close friend Kaherdin, lord of the city; his sister and Tristram’s wife Isolde of the White Hands; and Brangwen, La Belle Isolde’s faithful lady-in-waiting, are all surprisingly uncooperative. Entering a world of conflicting alliances and murderous gossip, Merlin and Gildas must work fast—or the next untimely deaths may be their own.