Dear Rich: I possess two letters written by a famous science fiction writer who lived in a foreign country and is now deceased. One is a copy of a letter written at my request to a postal official in Washington, D.C. The other was written to me personally. I contacted the literary agency that owns the rights to the author’s estate and requested permission to reprint the letters in a book. The agent for the estate in the U.S. responded that “Frankly, I am not comfortable granting permission to someone I don't know, who is not associated with a major publishing house or any other person or entity with a reputation that is known to me. Normally we just don't deal with individual persons unless they are referred or recommended by someone we know.” In fact, I had been referred to him by the literary agency that owns the author’s estate in Great Britain. Do I have the right to reproduce in my book a copy of the envelope that these letters arrived in? There is no copyright in a typical envelope (addresses and a cancelled stamp). The only possible claim that could arise would be -- and this seems like a longshot -- that if, by disclosing someone's address, you were invading their privacy. As for the letters, they are considered literary works and the writer retains copyright unless the letters were published before 1923, or published before 1964 but not renewed (in which case they are in the public domain).
Sending a letter to someone else transfers ownership of the physical letter, not the copyright. The science fiction writer's estate (or whoever acquired the writer's copyrights) can prevent duplication or further publication. You may be permitted to reproduce portions of the letters under fair use principles ... here are summaries of some cases.