The Question and Answer section for Atonement is a great resource to ask questions, find answers, and discuss the novel.
The typewriter, hence, the story Briony wishes to share.... symbolizes what could have been.... what Briony wishes would have been.
That is the main question of the novel. Does Briony finally achieve her atonement by writing her story and keeping her lovers and allowing their love to survive? I'm not convinced Briony does this. Briony is losing her innocence from the moment...
Check out this GradeSaver link.