The Order War by L.E. Modesitt, Jr.
"The deadly White Wizards of Fairhaven, wielding the forces of chaos, threaten the ancient matriarchy of Sarronnyn, the last bastion of order in Candar.
When Sarronnyn falls despite the assistance of the Black Order mages, Justen, a young Black Engineer, must escape the city and master forbidden technology that harness chaos itself. Even so, it may not be enough to halt conquest of the White Wizards."
Mass market, a nice copy!