The White Order by L. E. Modesitt Jr.

This is the story of Cerryl, a boy orphaned when the powerful white mages killed his amateur-magician father. Cerryl, raised by his aunt and uncle, is a curious boy, attracted to mirrors and books. The miller's daughter teaches Cerryl to read his father's books, and it seems that the talent for magic has been passed from father to son. When Cerryl and the miller witness a white mage destroy a renegade magician, the miller realises that he can no longer keep the boy safe. So Cerryl is sent to the city of Fairhaven to realise his destiny - to become one of the great magicians of his age.