I had a chance to visit the University of California Berkeley's Bancroft Library yesterday, which houses a collection of letters between Haas, the fantasist Clark Ashton Smith, and Smith's wife Carole. Apparently, the material was donated to the library by Don Herron in 2006.
I'll have more to say later, but one ironic bit: After Clark's death, Carole was looking for a repository for her husband's materials. She had some dealings with the Bancroft, didn't like the place at all, and swore that nothing of his would ever end up there.
Best laid plans and all that.