Wren to the Rescue

NY Public Library Best Books for Teen Age List, 1995
Finalist for Mythopoeic Fantasy Award for Children's Literature, 1995
Anne Spencer Lindbergh Honor Book, 1996.

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!