In looking for a means to review the important concepts and events depicted in John Steinbeck's The Grapes of Wrath, I would expect there are several important questions one might ask him- or herself in Chapters 17-21.
First, I believe it would be important to ask, "How is the importance of family in evidence in these chapters?" Family here applies not only to those related by blood, but also to the extended family made up of people who have joined together to travel westward in hopes of finding a better life.
Another important question would be directed to how the lawmakers fail to create and follow laws that protect everyone: "How are the laws followed by the police unlawful?" The laws established are only for the select few: those with land and money.
Another question might be, "Who would you describe as the "haves" and the "have-nots" in this story, and why is this distinction important? Obviously those who "have" do not want to share with the migrant workers (the "have-nots").
Another question might address financial issues: "How do the wealthy and the bankers control the economy in California, which directly affects the migrant workers who are trying to start a new life?" Keeping wages low and costs high controls the economic progress of the migrant workers; these issues also run small farmers out of business as well.