The Geography of You and Me by Jennifer E. Smith
"He was like one of her novels, still unfinished and best understood in the right place and at the right time.." Lucy and Owen meet somewhere between the tenth and eleventh floors of a New York City apartment building, on an elevator rendered useless by a citywide blackout. After they're rescued, they spend a single night together, wandering the darkened streets and marveling at the rare appearance of stars above Manhattan. But once the power is restored, so is reality. Lucy soon moves to Edinburgh with her parents, while Owen heads out west with his father.