I agree that the book doesn't have much to sink your teeth into. It paints a picture of wealth/influence, the haves and have nots, etc. that we have become really familiar with.
In this section, Lily realizes she screwed up with regard to Grce and that he will marry someone who doesn't even need his money which is ironic to Lilly. she realizes to what extent Bertha would go to to de rail her plans and thus when compromising letters fall into Lillies hands, she keeps them for possible use against her, in the future.