Never heard of this letter before, but it immediately made me think of Kenneth Patchen's The Journal of Albion Moonlight. To me it's a beautiful surrealistic masterpiece, written in the early 1940s, while Hitler was still alive. Hitler is a minor character, who memorably appears in a passage where he meets Jesus. I won't spoil it for you, but it still gives me goosebumps 25 years later thinking back on it. It's not a very long passage, but I think by itself it justifies reading the whole book, and I think it's more profound than Chaplin's The Great Dictator.