Sweet Black Waves by Kristina Pérez

As best friend and lady-in-waiting to the princess, Branwen is guided by two principles: devotion to her homeland and hatred for the raiders who killed her parents. When she unknowingly saves the life of her enemy, he awakens her ancient healing magic and opens her heart. Branwen begins to dream of peace, but the princess she serves is not so easily convinced. Fighting for what's right, even as her powers grow, will set Branwen against her best friend and the only man she's ever loved.

Inspired by the legend of Tristan and Eseult, this is the story of the legend’s true heroine. For fans of Graceling and The Mists of Avalon, this is the first book of a lush fantasy trilogy about warring countries, family secrets, and forbidden romance.