While doing some research on letter writing, I stumbled upon this fascinating collection of hand-illustrated letters from the Smithsonian's Archives of American Art. You can view all 178 of them here:

R. Lortac to Edward Willis Redfield, 1919

William Cushing Loring to Parents,1901

Pablo Picasso to Jean Cocteau, 1916

Alfred Joseph Frueh to Giuliette Fanciulli,1913