What are you learning about the culture or time period in which the Custom of the Country is set? How would you read this novel differently if you were living during this time period?

Undine Spragg is a young woman living in old New York in a very wealthy society where women's rights are still null. As such, Undine and other females do not have the chance to lead fulfilling lives that they would be happy with. Their "happiness" would depend almost entirely on their marriage prospects, namely, their husbands and, more importantly, how much money they had. 
Undine is well-aware of her social limitations, which is why she takes full advantage of her charms and uses marriage in her favor. She is not a charming person by any means, but her physical beauty, sophistication, and the speed with which she moves from social circle to social circle makes her quite appealing to an average man of that time and place. Moffat, for example, sees in Undine characteristics of wealth: flamboyance, extravagance, and snobbery. 
Indeed, extravagance and snobbery are typical traits in a big part of Undine's society. This is because the mega-rich families of old New York, like Edith Wharton's own family, existed as hierarchies that went back to the times of the early Dutch settlers. They acted like royalty, because they considered themselves to be the closest thing to it. Therefore, family name, family origin, and closeness to the Dutch heritage were highly-esteemed qualities that defined who you were in that particular society. This is why Undine had such a fixation with the Dagonet clan: They were all that, and more. 
Then there is the other side of Undine's society. Those who had neither name, fortune nor links to the Dutch were considered to be "new money." Moffat is a prime example of this. A "dealer," "trader," or salesman may come to money in old New York, but they would never be able to mix with the ancestral families. Undine was another example of new money. Her midwesterner family moved to New York to mingle in higher social circles. Undine knew well where she stood socially, and this is why she was so fixated in finding a label to identify her as something bigger and better. Hence, she marries Marvell in order to fit into an "old" family, then goes after Van Degen, then goes on to marry a French count, and, in the end, she ends up with Moffat because of all the opulence that he offers. Hence,Undine is eternally chasing 2 things that she lacks: social class and heritage. She lacks these two factors because she simply was not born into them. This is the other reason why she is consistently dissatisfied. 
Finally, in Undine's society, there has to be money as part of a successful social formula. Remember: family name, family fortune and closeness to their Dutch heritage were the ideal traits to have but, out of all three, the most powerful factor would still be money. In a society where the "new money" folk was getting rich enough to compete with the old money folk, those who lacked the class and heritage would resort to using their money to show it off in opulence. This opulence would mimic the extravagance of the rich, if not deflect the attention away from the old money family names and into the fantastic things that the new money could buy.
This is why Undine resorts to opulence and ends up with Moffat. Yet, still living in a mansion in Paris, Undine still feels empty and unhappy. This is is how important it was to live in old New York under "the right circumstances," and to be considered "somebody."
The modern reader would perhaps appreciate the social distinctions that existed then and still exist now. A female modern reader, however, would never be able be able to truly appreciate the extent to which women were so limited by a male-dominated society. True, there are changes that should still be happening in today's world with women equality, but the things that occur in Undine's world would definitely never be allowed to take place in today's law-regulated society.