Bridge of D'Arnath, book 2: Guardians of the Keep
Carol Berg2004

Seri has one errand to do as she waits for her beloved Prince of Avonar to regain his memory: to relay her brother's dying words to his son. But something is clearly wrong with the ten-year-old. Secretive, isolated, angry, the boy shows an inordinate terror of Seri's past. Though determined not to care for a child whose life she resents, she finds herself intrigued. And when an old nemesis arrives at her ancestral home...the world falls apart. A child with secrets, a sorcerer with a broken mind, a woman armed only with courage and wit, and an illiterate boy who knows everything of honor...all ensnared by the Lords of Zhev'Na in their plot to destroy D'Arnath's Bridge and plunge two worlds into chaos.