How can we to compare setting of the book to the setting which we live in 20th centry, for example, like the mood atmosphere or the setting?
The Lovely Bones is set in suburban 1970's Pennsylvania. This is a time that Susie Salmon describes as follows,
This was before kids of all races and genders started appearing on milk cartons or in the daily mail. It was still back when people believed things like that didn't happen.
This small description helps to establish the immense contrast between Susie's time, and our time. This is because, shortly after we read Susie's description in chapter 1, is when we realize that Susie's own disappearance will perhaps set the wheels of change in motion, and people "will believe" that things "like that" do happen.
If we were to compare Susie's time to our own, we will find that communication works as a factor of differentiation. Susie lives in a time where "milk cartons" are probably the only medium of communication to expose the social threat lurking beneath their all-American town. This is why, we see that Susie's parents do not seem too worried about everyday things such as trusting their neighbors enough to allow Buckley to go in their houses, or accepting food from Ruana Signh, nor letting their kids walk home from school.
The best asset that we have in the 21st century to use as a variable of comparison to the setting of the story in the 1970's is, undoubtedly, communication. It changes everything and sets the tone, atmosphere and mood of everyone's lives.
In the 21st century the media uncovers the horrors of crime from rape, to murder, to torture. We have a consistent "feed" of information telling us, or rather begging us, NOT to trust the potential "sociopath next door". We live in a hedonistic society where our constant self-exposure in the social media can make us prey to just about any unstable person out there. Our society is just as dangerous as Susie's, but we should (at least) know better. Susie's society would have never stood a chance during a crime wave, when their world moved as slowly as it did.
Furthermore, as far as mood and the atmosphere, The Lovely Bones, explores the shaking of the foundation of an everyday American family. Hence, the concepts of comfort, trust, and joy entail the achievement of the American Dream: one in which the home provides the safety and the community provides the support.
We also see the dynamics of everyday suburbia: The stay-at-home mom, the hardworking Dad who tends to his hobbies, the three children at school, the high school sweethearts, and most importantly, the seemingly-normal neighbor who tends to his flowers and shares tips with Mr. and Mrs. Salmon:
The murderer was a man from our neighborhood. My mother liked his border flowers, and my father talked to him once about fertilizer. [...] My father came home smiling, making jokes about how the man’s garden might be beautiful but it would stink to high heaven once a heat wave hit.
These details lead the reader to understand the real horror of Susie's death: How could a family so stable and normal, in such a normal neighborhood, undergo such a tragedy by one of their own neighbors?
Therefore, the main difference between that community and one today is that we have the benefit of communication and a strong media that exposes people for who they are. However, nobody never really knows who is who, for real. Like Susie says:
Don’t think every person you’re going to meet in here is suspect. That’s the problem. You never know.