Philip Reeve 
Here Lies Arthur 

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 Myrddin 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-taggle war-band into King Arthur, the greatest hero of all time.