Here is a book you can post. The dust jacket extends and wraps around, ready for a stamp and address.
It says: Pull out back flap, close book right up, wrap flap round book until gummed edge appears just below postage stamp, then stick down. The book is illustrated by Joan Hassell. Others were Old Christmases by William Strode, illustrated by Anthony Gross.
In the communication of today it seems so alien to make the dust jacket part of the design for an envelope, but I think it’s one of the most charming things I have seen.