The Wild Queen - The Peasant Queen Series, #2

Roweena is the crown princess of Norvallen, a tiny kingdom with only one thing of value-the Healer's Grove. The trees in this small section of forest are enchanted, giving a sap that can be mixed into potions or salves to heal almost any wound. Lucien, the young king of neighboring Demarde, comes to Roweena's father seeking an alliance, but comes away with a marriage contract for young Roweena's hand.
Furious and stubborn, this untamed beauty vows he will never conquer her. Before Lucien can return, Roweena's home is attacked and her parents are murdered. With nothing more than her horse and the clothes on her back, Roweena goes to the only person she knows can help her. Lucien.