Found: This book was The Broken CitadelThe Broken Citadel by Joyce Ballou Gregorian, and it's the first in the Tredana trilogy.
In the Otherworld, change was afoot. The winds whistled through the ruins of Treglad, echoing the name of a long-forgotten goddess. In the city of Treclere, the Deathless Queen slowly drew all the land under her spell of dark sorcery. In the city of Tredana, a Prince pledged himself to a quest that would make his destiny--or would take his life. In a tower on an island of glass, a Princess was imprisoned by her own mother's decree. In the ice of the north the Kermyrag burned in eternal flames, with a crown between his white-feathered wings and tears of blood in his dark eyes. And in Massachusetts there was a window to Otherworld, and a young woman named Sibby was about to step over the sill.
I now have a copy on order to see if it will still resonate five decades later.
