Lee's only novel, To Kill a Mockingbird (1961) has been read by more people than perhaps any other book our group has read previously (30 million copies sold), with the possible exception of Lolita. Consequently, it is almost impossible to say something unique about the novel so I am going to address some of the less noted aspects of the book.
Mockingbird is not a perfect book and has its detractors. Some of the less flattering commentary has been superficial criticism based on Lee's use of the epithet, "nigger." Other critics have complained about the fact that one of the characters, a white woman, was attracted to a black man, then falsely accused him of rape. Those kinds of reactions are to be expected when a novel of this caliber addresses issues such as race and class, and does so in a divisive, but insightful manner.
Our group did not waste any time re-hashing these sorts criticisms that one expects to hear from readers who prefer inoffensive, non-controversial literature. There are, however, some other points worth mentioning. Specifically, there are a couple of characters who have appeared elsewhere in the world of literature. The first character is Calipurnia. It is possible to see the her as nothing more than the stereotypical Wise Black Servant (similar to Dilsey in The Sound and the Fury - Wm. Faulkner, 1929).
The second character is Boo Radley. He is the Local Monster, the ogre inhabiting a small town who is feared and avoided by all the children. This character has also been used by a well-known writer other than Lee. In 1946, the first copyright of Ray Bradbury's Dandelion Wine (by Bradbury) appeared, then again in 1957 by Doubleday, preceding Mockingbird. Set in Waukegan, Illinois, Bradbury's local monster was referred to as "The Lonely One." Lee's small town, Monroeville, Alabama, featured Radley, a mysterious man who lived near the protagonist and narrator, Scout. Both the Lonely One and Radley were, at least initially, blamed for attempting to victimize unsuspecting females. To Lee's credit, though, there is an important difference between the two bad guys: Radley turns out to be heroic. Lee's transformation of Radley was a nice, surprising touch.
(Note to aspiring novelists: if you are in the process of writing a book set in a small town south of
the Mason-Dixon line, and there is a character who is a black female servant,
please do not turn her into Dilsey or Calipurnia. Nor should you include a local monster in your cast. The exception, of course, is if your black female servant becomes a brain-eating zombie vampire. Then, you are headed for the best-seller list.)
Radley's heroism, surprisingly, becomes necessary because Scout's father, Atticus Finch, a well-regarded local lawyer, has misjudged the seriousness of the threat posed by another character in the book. It is surprising because trial lawyers earn their living, in part, by becoming excellent judges of human character. Despite Atticus' ability to expose lies spoken on the witness stand, he is unwilling to recognize that a character poses a real risk to his daughter's safety. The irony of Atticus' failure to see this danger sets up a superbly tense climax to the story. He won't acknowledge that, sometimes, 1) there are a few people in small towns who are truly evil; 2) children can have more accurate insights into human nature than adults; and 3) the smartest guy around can err in his judgment.
These observations about the nature of humanity, beyond class and racism, make Lee's book truly literary and a valid starting point for discussion, regardless of whether the work is assigned reading or simply a great book you have always heard about, but never read.
Lastly, To Kill a Mockingbird was made into a film starring Gregory Peck, who won an Oscar for his portrayal of Atticus Finch. I have not seen the film, but the author is said to have been pleased with it.