A Wizard Abroad — Diane Duane

To give Nita a vacation from magic--and to separate her from Kit, her partner in wizardry--Nita's parents pack her off for a monthlong stay with her eccentric aunt in Ireland.

But the Old World is even more steeped in magical doings than the United States, and Nita soon finds herself and a host of Irish wizards battling creatures from a nightmare Ireland -- a realm where humankind is the stuff of tales and storybooks, and where the legends and monsters of Irish mythology are a deadly reality.

So much for a vacation abroad....