Essentially this is the story of Si Morley who is chosen by a secret government project that is researching the possibilities of time travel into the past. Plucked from his hum drum life, he accepts without knowing anything about the project before hand but welcomes the chance to give his life more meaning.
One might be tempted to think that the theme of this book makes it SF territory but the rationale and logic behind the time travel mechanism doesn't really bear much scrutiny. It is vaguely explained to Si that it is sometime to do with an extrapolation of Einstein's theories of space/time and what's essentially required is that one untie all the threats of thought and belief that tie one to the present moment and then one can simply step into another time period. Quite an interesting notion but it isn't really explained much better than that and that's because the author had far less interest in the process and far more interest in actually being in the past.
The place is New York and the time is the early 1880's. In reading this I can only imagine that he as extensively researched this time period because the book contains detailed descriptions of New York, the buildings, the people and the way of life back then. Even included are many old (but genuine) photographs and sketches of New York at the time and are included as part of Si's first person account of his travels.
He goes back several times, strictly to observe but gets more and more drawn into events and involved with people of the time. On his return the project attempt to assess whether he has changed the future in any way by getting him to run off all the general knowledge facts he can remember. But eventually it turns out that they want more than a deeper understanding of past events...
If you are familiar with New York, even have a keen interest in the place, you will love this book. I've never been to the place and so much of the detailed descriptions of particular avenues, cross streets, buildings and aspects went over my head. I think the author set out to really convey a sense of wonder at really being there and to bring that time and place alive. In this I think he succeeds. The pace of the plot is innevitably drawn out in the process but I didn't mind too much, despite not having a particular interest in the place/time in question, because he did such a good job of transporting you there. It is fascinating how rapidly this city developed and how different a place it was just over 80 years before this book was written (around 1970).
Jack Finney undoubtedly had a rose tinted view of the past but not too a fault. The severe poverty of the poor, the long working days, the diseases and illnesses that ravaged us then and the corruption that plagued law enforcement are all brought to the reader's attention but one is definitely left with the impression that, all in all, the past was a better time.
But all in all a very good story if you are okay with it's what the author was trying to do (and what he wasn't).