Kincaid has created a wonderful regional story in the tale of this young protagonist.  However, beyond that, she provides readers with an in-depth look a typical teenage girl looking for her identity.  Once she hits puberty, Annie seeks independence.  She rebels, challenging her mother and her mother's rules, and trying to identify who she is and what her own priorities are. 

Annie's stealing is an attempt to do this.  Her interactions with the Red Girl allow her to explore the forbidden, from something as simple as playing marbles to the more devious stealing of books and stashing of 'treasure'.  Her guilt extends only so far as her belief that her mother understands her.  When her mother begins telling Annie of her own experiences as a young girl, Annie starts to feel a connection with her mother again.  She feels shame and wants to confess.  However, that feeling immediately evaporates when Annie understands that her mother is only attempting to trick her into a confession.  She doesn't see the stealing as 'theft', but as a necessary rebellion.

There have been other questions similar to this.  I've included the link to those below.