Why does Harper Lee split the book into two parts and why does she include the information she does in part one?

Part 1 of To Kill A Mockingbird
focuses mainly on the children's obsession with Boo Radley.  In the novel's early
chapters, readers come to recognize Scout, Jem, and Dill as curious children who
entertain themselves by trying to get a glimpse at their mysterious neighbor.  Despite
Atticus's advice to leave Boo alone, the children keep at their games and fail to
understand the fact that their behavior is hurtful and
unfair. 

In Part 2 of the novel, Harper Lee shifts her
focus to the trial of Tom Robinson, which is a clearer example of the social (in this
case racial) injustice that was all too common in the 1930s in the South.  Watching
their father defend Tom Robinson, who is obviously innocent of the crime of which he's
accused, teaches the children that it is unfair to prejudge otherse and that a person's
social/economic status does not dictate his or her worth as a
person. 

At the novel's end, and after having been saved by
Boo Radley, Scout is able to finally understand Atticus's advice that we can never
really understand others until we put ourselves in their situation.  The novel's two
distinct focuses (the children/Boo Radley in Part 1 and the Tom Robinson trial and its
fallout in Part 2) serve to teach readers that injustice is present in all facets of
society, and Lee successfully brings these ideas together at the end of the
novel.