Vincent van Gogh is famous for the nearly 900 paintings he produced in his ten short years as an artist. His sketches and watercolours, too, have become extremely well known. It's important to remember, however, that Van Gogh produced works no less remarkable in yet another medium: letter sketches. Throughout his career as an artist Vincent maintained a warm and loving correspondence with his brother, Theo. In many of the letters to his brother (and others as well) Vincent drew sketches within the letters themselves--as a means of showing how his work was progressing.
A nice collection at Bibliodyssey
And all 133 are available at vggallery.com