(Incidentally, there's also a close connection between the historical Hannibal and Campania, where the Amalfi Coast is located.)
There was an exhibition at Van Gogh Museum, Amsterdam, 'Van Gogh's letters: The artist speaks' (9 October 2009 - 3 January 2010).
Available is the landmark six volumes of 'Vincent van Gogh - The Letters: The Complete Illustrated and Annotated Edition' (Thames & Hudson, 2009).
Many years ago, some letters of Edouard Manet with fanciful flourishes of watercolor sketches made quite an impression on me.
There are also a few books on the illustrated letters of artists and writers.
There are endless possibilities for artful letter writing, which can be formal or casual. The important thing is not to so quickly succumb to the predominant functionalism.