The book talks about ancient cultures in the U.S. It includes the Adena culture which went back to around 1000 BCE, the Hopewell culture which went back to 200 BCE. the Mississippian culture, going to 800 CE and having large urban centers, Woodhenge, Poverty Point and other places.
The importance of the book lies in its showing that just became cultures might be old and no longer around doesn't mean they were dumb. All these cultures were organized, knew astronomy, were capable of developing cities, had trading systems and social stratification. The trading systems sometimes involved very long distances, also.
Ohio seems to have various cultures over time, also.
I think it's very important to realize that just because a group of people existed a long time ago doesn't mean they were ignorant. They lived much like us today but without the technological emphasis.