Lee’s first submitted version of the work was reportedly more of a series of linked stories (ironically, a very hot form in fiction at the present moment.) But her publisher insisted on a more unified novel.
Lee was able to achieve this partly by following in linear time the education of the young girl, Scout and her brother, Jem, and partly by weaving a few important themes throughout the book. The first was about innocence, both of children and of “the mockingbird”—the innocent who only sings and does no one any harm—which applies to both Tom Robinson, the accused black man Atticus defends, and to Boo Radley, the neighbor who lives in darkness, the stranger in their midst who receives their projections of violence, and is therefore a source of fear.
He is different (and a kind of artist, who creates sculptures and leaves them for the children to find, along with talismans of his own “normal” childhood). He is literally unseen, and so represents the aspects of people we are blind to, because of our preconceptions. This obviously applies to race, and there is also a strong theme of class in the novel—which cuts both ways. (It can be argued that Atticus has his own class prejudices.)
The second theme, which follows from the first and is explicitly stated as a lesson to the novel’s children, is that of cultivating empathy and understanding by trying to see the world from the other’s perspective (as Scout does finally when she stands on Boo Radley’s porch at the end), by metaphorically living in someone else’s skin, walking in their shoes. This is a lesson about life and specifically about race. It remains the most crucial lesson in our public as well as private lives, and so this too accounts for this novel’s standing.
It is reinforced in other ways throughout the novel, notably by the brief story of Mrs. Dubose, a surly neighbor who insults everyone, including Scout and Jem, and says harsh things about Atticus. When Jem loses his temper and destroys her garden flowers, Atticus sends him to Mrs. Dubose to apologize and make restitution. Mr. D. requires him to read aloud to her everyday. When she dies, they learn that she was always in pain and addicted to morphine, which accounted for her harsh behavior. She decided she would die free of her addiction, and Jem reading to her was a way for her to bear the pain. It’s another example of assumptions and projections contradicted by understanding, as well as a story of redemption and the power of simple acts to do good.