Any books on why an older teen would want to be adopted?

It seems there are a lot of books to explain the adoption of infants and young children and how to integrate them into families. Are there any that explain why an older teen or young adult would long to be adopted? I am the only one in our family, immediate and extended including an adoptee, who understands why our son who came to us at 17 in foster care would like to be adopted legally. They won't get on these boards, of course, and I can't seem to make them understand, but if there were a book that explains how it feels to be finally adopted at such an "advanced" age, maybe they would read it. Thanks.