On one level the book is a kind of romance. Because a lot of the drama turns on the difference between the haves and the have-nots in the United States - where the former are the metropolitan elites and the latter are the religious rural folk in the red states - the prize in the end turns out to be something different from marriage. Marriage for Dellarobia having always signified failure and compromise. One of the agents of change that comes into Dellarobia's life is a scientist, Ovid Byron, who has arrived at her doorstep to research the butterflies. His presence turns out to have other implications as well.
Another strange agent of change for Dellarobia - who has a dutiful and sensitive young son, Preston, and a rowdy infant daughter, Cordelia - is Hester, her mother-in-law. At the beginning of the book Hester is the face of censure and disapproval in Dellarobia's life, a source of danger, and someone who she has to navigate around, like a reef for a frequent sailor. It is later in the book, when Dellarobia has had chances to deeply interrogate Hester's life - she finds out the older woman had had a child given up for adoption before Cub, Dellarobia's husband, was born - that things start to get out of control.
But out of control can be a good thing when your life is stuck in a rut and you don't love your husband, although you may respect him regardless. Dellarobia is an intelligent woman who never had many opportunities given to her, and she is in the habit of asking "why" at times when other people might take home truths for gospel truths.
The book is filled with small events and is peopled by strange characters the author handles with complete aplomb. Kingsolver is clearly a writer who is used to being in control of her material.