The Dreamstone — C.J. Cherryh

It was that transitional time of the world, when man first brought the clang of iron and the reek of smoke to the lands which before had echoed with inhuman voices. In that dawn of man and death of magic there yet remained one last untouched place -- the small forest of Ealdwood -- which kept a time different from elsewhere, and one who dwelt there who had more patience, pride and love of the earth than any other of her kind -- Arafel the Sidhe.

This is the story of the last defense of Faery against the encroaching iron sword of the Era of Man.