Postcard sent by Bryon (the USA). I wrote about this book long time ago, and I was more than happy to receive this postcard!
The Jolly Postman (1986) by Janet and Allan Ahlberg is a very special interactive book. The plot: a mail carrier delivers letters by bicycle to several famous fairy tales characters. Twelve of the pages have been made into six envelopes. They contain the real letters and cards that may be removed from their envelope page and read separately.
There are two sequels: The Jolly Christmas Postman (1991) and The Pocket Jolly Postman (1995). I have read only the first. Actually, I do not own the book myself: I have bought more than once, but I always finish to offer it as a gift.