The chapter that would be most important would have to be chapter thirteen. Without reading this chapter a reader would have no knowledge of a key event in the storyline, the death of Granpa. There are several ongoing themes throughout this chapter, but the most prevalent seems to be the importance of family. The book spends a great amount of time going into detail about the various family functions, how everyone has a job and a place in the group's pecking order. It comes back to describing family meetings as almost political from previous chapters, reinforcing the importance of tradition and the family's social order.
"I seen fellas like you before. You ain't askin' nothin'; you're jus' singin' a kinda song. 'What we comin' to?' You don' wanta know." (Steinbeck 128)
Has nothing to do with what I talked about before, just found the passage thought provoking.