The Sword of the Lady, Paperback/S. M. Stirling

Rudi Mackenzie's quest to find the source of the world-altering event known as The Change ends in Nantucket, an island overrun with forest and inhabited by a mere 200 people who claim to have been transported there from out of time. Only one odd stone house remains standing. Within it, Rudi finds a beautifully made sword seemingly waiting for him. And once he takes it up, nothing for Rudi or for the world that he knows will ever be the same...