Read other letters about this article
1. The book absolutely needs readers' letters too.
2. Include a few columns where you screwed up with some lousy advice (not hard to find), and much better advice was forthcoming from readers
3. Some meta-comments would be great - about the Big Themes from advice seekers (I screwed up my life 20 years ago/my parent hates me/my best friend eloped with my husband/etc)
4. Some meta comments about giving advice.
You definitely need more material than just original letter + your reply.