What is the true meaning of the novel

This question is certainly thematic in nature. I'm going to go with the obvious which is guilt and atonement. 

The theme of guilt, forgiveness, and atonement should be extremely obvious to anyone who reads the book. The entire plot of the novel centers on a woman who devotes her entire life repenting a crime she committed while still a young girl.

Articles of note that are not as obvious to the reader that have to do with this theme are things like, is Briony the only person who should feel guilty? Who else is at fault for the crime committed on that hot summer night in 1935? Where is Lola's guilt for not saying anything? What about Paul Marshall's--the real assailant who gets away with rape and stands silent while an innocent man goes to prison. Then there are all the adults in Part One of the novel. How is it that so many people who are capable of understanding so much more than a thirteen-year-old girl come to rely completely on her testimony? Should more not have been done in the investigation?

The question is left open at the end of the book. Does Briony finally achieve her atonement by writing her story and keeping her lovers and allowing their love to survive?

The second layer to the guilt theme has to do with the history of literature. Aside from the crime she committed as a child, Briony feels guilty for her powers as a writer. She knows she has the autonomy to write whatever story she so chooses. Just like she could send Robbie to prison, she can make him survive the war. The reliance readers put in Briony to tell them "what really happened" leaves her feeling guilty about her life's work, and she projects that guilt onto the history of the English literature canon. You can find more themes at the GradeSaver link below: