What was the writer's purpose of the book "The Face on the Milk Carton"?

To inform, persuade...? What and why?

What was the writer's purpose of the book "The Face on the Milk Carton"?
Oh that is weird, I just finished that book! I'm not really sure what the purpose was exactly but maybe the author just wanted to write a good mystery. Sorry my answer wasn't a very big help but did you like the book?