The Spellsong war - L. E. Modesitt Jr.

When Anna Marshall wished she could be anywhere but Ames, Iowa, and anything but a singer and music instructor at Iowa State University, she did so at exactly the wrong time and found herself pulled from Iowa to the very different world of Erde. On Erde, music is magic, and Anna's ability makes her an enormously powerful sorceress - potentially.

With her unique power, Anna saves the kingdom of Defalk from invasion and in six months becomes its regent. After six years of war, Defalk is weak and Anna must turn her efforts to rebuilding the kingdom. But she has little time: Defalk's southern neighbour is already encroaching on the border. Rising to power in Defalk may have been the easy part; now Anna will need all her skill merely to hold on to what's she's managed to accomplish so far.