Last year / Robert Charles Wilson.

"A Tor Book."

"Two events made September 1st a memorable day for Jesse Cullum. First, he lost a pair of Oakley sunglasses. Second, he saved the life of President Ulysses S. Grant. It's the near future, and the technology exists to open doorways into the past--but not our past, not exactly. Each "past" is effectively an alternate world, identical to ours but only up to the date on which we access it. And a given "past" can only be reached once. After a passageway is open, it's the only road to that particular past; once closed, it can't be reopened. A passageway has been opened to a version of late 19th-century Ohio. It's been in operation for most of a decade, but it's no secret, on either side of time. A small city has grown up around it to entertain visitors from our time, and many locals earn a good living catering to them. But like all such operations, it has a shelf life; as the "natives" become more sophisticated, their version of the "past" grows less attractive as a destination. Jesse Cullum is a native. And he knows the passageway will be closing soon. He's fallen in love with a woman from our time, and he means to follow her back--no matter whose secrets he has to expose in order to do it"--