Title: The Bradleys and the mysterious letter
Description: From the cover: "Mystery and adventure flow from the pen of Bernard Palmer in this story which introduces the Bradley family.
Probably best know for his Danny Orlis series..."

Written in 1975. I only read the first chapter and it appears to be in that time period.  The Bradley family is living in the Detroit area; they moved there only 2 years previously because of the dad's job.  And now he has another job offer in Nebraska and the kids are upset at the idea of moving.