Duel of Dragons (Dragonsword #2) by Gaèl Baudino

Wings of Doom were spreading destruction across the kingdom of Gryylth. Twentieth-century weapons were raining down upon an unsuspecting land where sharpened blades and pointed spears had always been the only true warrior's tools. And for Suzanne Helling, UCLA teacher of medieval studies, the call of the dragon Silbakor has suddenly come again, transforming her into Alouzon Dragonmaster, Guardian of the land. But even as Silbakor prepares to return Suzanne to Gryylth, they are attacked by another dragon with enough power to rival Silbakor himself. This creature of evil was ready to pursue them all the way from Los Angeles to the very heart of Gryylth. And once returned to this land originally conjured by the imagination of a now dead professor, can Suzanne find and defeat this nameless, faceless enemy?
(from goodreads.com)