Here Lies Arthur By Philip Reeve:

(Used, paperback)Gwyna is just a small girl, a mouse, when she is bound in service to Myrddin the bard - a traveller and spinner of tales. But Myrdin transfroms her - into a lady goddess, a boy warrior, and a spy. Without Gwyna, Myrddin will not be able to work the most glorious transformation of all - and turn the leader of a raggle-tagglear-band into King Arthur, the greatest hero of all time.