Answer:
But the mark of a truly great novel is that it exists, or can be read, on a number of different levels of meaning. These levels could also be understood as themes, and To Kill a Mockingbird has many themes. In short, the novel has many ideas for the reader to think about as she reads. Here is a partial list, in no particular order of importance:
a picture of the American South in the mid-twentieth century
fascinating characters and character development
family dynamics
race relations
justice
the outcast
appearance vs. reality
coming of age
education