Time and Again by Jack Finney

When a secret government study group explores the possibility that Einstein's theory of time means history is as much a matter of where as it is of when, Si Morley finds himself with the ability to leave his room in the Dakota of 1970 and walk out into the New York City of 1882.  Hoping to solve a family mystery that has haunted his current girlfriend, Si finds himself caught up in political fallout of the Boss Tweed era.  The time travel adventure is kind of cool but the picture of old New York is wonderful for anyone who knows the city.