The Bleak and Empty Sea: The Tristram and Isolde Story — Jay Ruud

When word comes to Camelot that Sir Tristram has died in Brittany of wounds suffered in a skirmish, and that his longtime mistress, La Belle Isolde, Queen of Cornwall, has subsequently died herself of a broken heart, Queen Guinevere and her trusted lady Rosemounde immediately suspect that there is more to the story of the lovers' deaths than they are being told. It is up to Merlin and his faithful assistant, Gildas of Cornwall, to find the truth behind the myths and half-truths surrounding these untimely deaths. They take ship to Brittany to investigate, and find themselves stymied by the uncooperative attitudes of 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.
The case is complicated by the facts that King Mark of Cornwall is Gildas's own liege lord, and that Duke Hoel, Lord of Brittany, is King Arthur's close ally and father of the lady Rosemounde, who urges Gildas to clear the name of her half-sister, Isolde of the White hands, whom gossip has implicated in Tristram's untimely death. By the time they are finally able to uncover the truth, Gildas and Merlin have lost one companion and are in danger of losing their own lives.