The Van Gogh Museum in Amsterdam has made it possible for us to view the actual letters of Vincent Van Gogh to his brother Theo, in Vincent's own handwriting, complete with sketches.
This is endlessly fascinating stuff to me as I have already read Van Gogh's letters over and over again for inspiration ("Dear Theo", a book edited by Irving Stone). It's amazing to see the preliminary sketches (leading up to well-known paintings) that he included in these letters from 1874-1890. There are also letters to Paul Gauguin and Emile Bernard (see the top image with notations about colors). There are a handful of letters from Gauguin himself that include important sketches as well!
For another blog post on this topic: BibliOdyssey