Arrow's Flight by Mercedes Lackey is a fantasy novel set in the Kingdom of Valdemar. It is the second novel in a trilogy of which the protagonist is a young woman named Talia.
In the universe of the novel, two types of magic exist, the sort of true magic we find in most sword and sorcery novels and a second type called "mind magic" which involves telepathy and other similar powers of the mind. The land of Valdemar is a kingdom run by a monarch with the help of a special group of people known as "Heralds", who are "chosen" by magical, quasi-divine horses called "Companions" who are intelligent and communicate telepathically.
Talia is a Herald from a small village who was chosen as a Herald in the previous book in the series, and as a special duty ended up taking care of the heir to the throne who was being corrupted by an evil magician. In this book, the heir Elspeth redeems herself and becomes a Herald and Talia continues to suffer prejudice because of her humble origins. The main conflict of the story involves Talia losing control of her telepathic gifts and having to retrain them.