Wren to the Rescue — Sherwood Smith

All her life Wren has hoped for an adventure. Now she has one—with a kidnapped princess, a handsome prince, and a magician. What does it matter if the princess is only Tess, her best friend from the orphanage; if the prince is a youngest son with no chance of becoming king; and the magician is an apprentice? Wren leads the other three over mountains and past killing spells, fighting battles along the way. But then she finds herself up against some shape-changing magic that may end her life as a human forever!