A few people were lucky enough to be pan pals of a sort with Vincent Van Gogh. Van Gogh often added sketches or paintings to his letters, to illustrate what he wrote about. BibliOdyssey has a collection of these letter sketches, along with the letters that accompanied them. Link
This publication is accompanied by a beautiful exhibition in the van Gogh museum in Amsterdam.
A friend of mine often writes on a notecard that she's illustrated with a pen and ink (sometimes watercolor) sketch... I save them!
Just amazingly beautiful. Thanks a lot for the link.