If you are looking for good types of books to introduce writing letters to pen pals, here are a few good books that I have used:
Donovan's Word Jar, Communication (Mulberry Books), Speaking Through Pictures (Hello Out There), What Was It Like Before the Telephone? (Read All About It), Kids Make a Difference: Kids Communicate, Pen Pals (Gazelle Books) by George Craig, P.S. Longer Letter later by Paula Danziger, Snail Mail No More by Paula Danziger,
Hope this helps!