I'm not so keen on the Hannibal Lecter films, but in the second installment Hannibal (2001), I do like the segments of the letter to Clarice. It does get one thinking about the dying art of letters. Who writes letters on perhaps Amalfi stationery finished with a wax seal anymore, with calligraphy and personalized art?
(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 Édouard Manet with fanciful flourishes of watercolor sketches made quite an impression on me.
Consider getting this book, More Than Words: Illustrated Letters from The Smithsonian's Archives of American Art (Princeton Architectural Press, 2005) which was supplemented by an exhibition of the same name - see above link.
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.