The novel is an excellent vehicle for social commentary. Consider Charles Dickens, Mark Twain, and John Steinbeck. All wrote books in order to bring social justice issues into the national conversation. Charles Dickens began his career as a court reporter and journalist. He saw a system of unfairness. He saw people suffering from laws and policies, and others ignoring them. People might be able to pass a beggar on the street with little qualm, but what about Tiny Tim and Oliver Twist?
As David Purdue pointed out, Dickens particularly attacked Britain’s Poor Law. The Poor Law developed workhouses to feed the poor and put them to work. In theory, it would keep the poor fed and housed. The reality was quite different.
Dickens, because of the childhood trauma caused by his father's imprisonment for debt and his consignment to the blacking factory to help support his family, was a true champion to the poor. He repeatedly pointed out the atrocities of the system through his novels. The full page can be found here:
How did Dickens accomplish this remarkable feat? He gave people story lines as contexts for characters they could relate to or sympathize with. Who hasn’t wanted to catch a break like Mr. Micawber? Who would deny help to Tiny Tim? Don’t we understand what made Oliver Twist turn to a life of crime?
In his plot lines, Dickens does not create a circumstance where we sympathize with the perpetuators of the system. We sympathize instead with its victims—the same people that we would normally walk by. Consider this exchange between the undertaker and beadle in Oliver Twist.
'The prices allowed by the board are very small, Mr. Bumble.'
'So are the coffins,' replied the beadle: with precisely as near an approach to a laugh as a great official ought to indulge in. (enotes etext p. 16)
The heartless exchange, laughing about how thin the poor became and how many coffins they needed, hit home for Dickens readers. Dickens's novels were instrumental in repealing the Poor Law. How many novels can say that?
Unlike newspaper articles, novels draw the reader in. They create a world in which characters we care about exist. Dickens made sure that the world mimicked reality, so his readers would begin to care about that too. It is no coincidence that Oliver, Tim and Scrooge are still well-known characters. The issues still exist, to a certain extent.
Here are more references for you:
Charles Dickens biography and historical context: http://www.enotes.com/oliver-twist/author-biography
Oliver Twist etext http://www.enotes.com/oliver-twist-text
David Purdue. "David Perdue's Charles Dickens Page - Dickens' London." David Perdue's Charles Dickens Page - Dickens' London. David Purdue, 1997. Web. 05 May 2012. <http://charlesdickenspage.com/dickens_london.html>.
Good question. Look at Animal Farm. Orwell writes this attack as an allegorical fairy tale to make it "instructive." On the other hand, Wiesel's Night is poetic to give it more emotional impact. Steinbeck's Grapes of Wrath alternates chapters of traditional narrative with descriptions of the land, in which the land almost seems like a character itself, for the purpose of emphasizing the importance of the soil and the environment.
These formal aspects help the writer's make their "social" comment.
I'm not thinking about any particular book, but the novel genre in general.