A sorcerer's treason - Sarah Zettel

Volume one of the stunning fantasy trilogy set partly on the shores of Lake Superior in the 19th century.

Bridget Lederle is the lighthouse keeper of Sand Island Lighthouse, a woman ostracised by the locals because of the bastard child she bore to a visiting Finnish fisherman, and because of her own mysterious antecedents. It is rumoured she's a witch; certainly, she has strange and uncontrollable visions and premonitions. One day, she has a premonition of disaster and 'sees' a small boat being dashed to pieces on the rocks off the island in a storm; the next day the storm kicks up - and she goes out to find a man thrown up on the shore, almost dead. When he speaks, it is in an unknown language; he has bizarre tattoos, and when he wraps a piece of braided cloth about her arm, she is suddenly able to understand him.

He tells her he is from another world, a world in which she is a great magical force, and that her presence is required to save the Dowager Empress, currently under threat from her poisonous daughter-in-law. It is an attractive offer for a woman with nothing to lose (her baby died, she has no prospects in our world); she goes with the wizard to Isavalta. But there she will find nothing is quite as he promised her.