One of the biggest differences between the book and the show is the debutante ball Belly participates in, which is not a plot line in the book at all.
On the show, Susannah encourages Belly to participate in the debutante ball, much to Laurel's chagrin. Conversations about the ball are interwoven throughout season one, from who will escort Belly (Susannah asks Conrad to, but Jeremiah takes her instead) to Steven losing all his money at a poker game and not having money to buy a matching tux for his escort. Things come to a head at the ball when Jeremiah learns that Susannah's illness is back. Struggling to deal with his emotions, he leaves Belly without a dance partner, and Conrad swoops in.
When the truth about Susannah comes out at the ball, it shakes their blended family and leaves them struggling to figure where to go from there.
In an interview with The Wrap, Han talked about the significance of the deb ball to the story. "The Deb Ball was a chance to really bring a rite of passage like a ceremonial rite of passage of growing up to life — to really see that visualized, because I think different cultures have many different ways of marking that moment of between, like, girlhood and adulthood," she said. "And that's what Deb Ball is, you're coming out and being seen as an adult."