Word of Advice: don't let your kids know you are about to try and get them 36 books, odds are, at least one of your well-meaning friends will be like me and forget to send the letters out. I DID send a book to the kid in #1 though. I think I will send the letters some day, so my friend's kid might randomly get books in like... a year. Plus I up and created and illustrated a letter that wasn't somewhat manipulative. If you never got that version, check out this blog. She quotes the original and then adds her own awesome twist.
Second Word of Advice (updated July 20, 2014): I was informed by "Legal Mind" in the comments that chain mail and pyramid schemes are fraudulent. While I feel only a wet blanket would actually take a person to court over this one in particular, I must advise that you get permission before sending the letters. Though we aren't fans of them, wet blankets and humorless, spiteful people both exist. BEWARE!
Book Exchange Letter
the only chain mail / pyramid scheme that won't leave you feeling like a fool