I read this book in six grade in the mid-80s.  I think it was a fantasy type story.  This society had once been in contact with humans (or maybe just one human).  He may have come over the mountains on a hot air balloon.

They got a few things confused about humans.  For example, they had a painting of a tree and a diagram of a family tree (genealogy chart).  They called the genealogy chart a picture and the painting of the tree a family tree.  So everyone in this society had a very carefully tended “family tree” in their yard.  Unfortunately, I remember nothing about the actual plot.