Guys, I don't think Leon is asking whether we would like to see more of these docs, but rather what are the legal ramifications (if any) of publicly posting what was originally private correspondence.
My completely non-lawyerly opinion would be that the posting of letters related to hobby matters and reasearch on a message board dedicated to hobby matters and research would fall under "fair use." My understanding is that whoever wrote the letter would technically retain copyright, but as long as you're not profiting from posting the letters and not hampering the author's ability to profit from his "creation" (which doesn't appear to be the intent of the letters in the first place), it should be okay. I found a pretty good (at least to my quick read through) explanation of "fair use" here:
http://www.citmedialaw.org/legal-guide/fair-use
Hope that helps, and by all means, if anyone reading this is actually trained in the law, please chime in and/or correct me.