Emily Post started writing about etiquette the decade after waves of immigrants landed on Ellis Island. These immigrant families, anxious to assimilate into America and join the middle class, used her writings to familiarize themselves with American social customs. They were also politically progressive, so it makes sense that they admired both Post and Eleanor Roosevelt. I suppose someone like Martha Stewart plays the role today that Emily Post had in the early and mid-twentieth century.

Very good point. The key development was publishing the rules of etiquette in a book that anyone could read (although Mrs. Post wasn't the first to do that.) In the old days, upper-class people learned the rules from their mothers and grandmothers.

Although I question whether anyone actually went from Ellis Island to the kind of world Mrs. Post describes. Not in less than a full generation, in any event. But by the end of WWII, the old barriers were breaking down and that world would never be the same again.