Still not an actual law, but this is interesting:
“Allegations involving copyright infringement frequently occur when the author of an unauthorized biography makes use of the subject’s published or unpublished letters and papers or possibly from oral conversations the author may have had with the subject.
In Salinger v. Random House, Inc., the author’s use of extensive quotations from unpublished letters written by J.D. Salinger, the subject of the biography, without Salinger’s permission was deemed to be copyright infringement. Under copyright law the writer of unpublished letters has the right to control the first publication of those letters.
Criticism of the Salinger decision as well as other copyright infringement decisions based upon what was believed to be excessive protection for unpublished works, resulted in Congress amending the Copyright Act. U.S. copyright law now provides that if a work is unpublished and it is used without permission of its author then the fact that the work was unpublished by itself would not be sufficient to constitute copyright infringement.
Subsequently, in Wright v. Warner Books it was held that when a work is one of “criticism, scholarship or research,” that quotations from unpublished personal letters and journals might constitute fair use.
In Estate of Hemingway v. Random House, Inc., which concerned the publication of portions of conversations between the author and Ernest Hemingway, the court ruled that the author of Papa Hemingway did not infringe the common law copyright of these conversations by including them in a book on Hemingway’s life. The court stated that even if the author used verbatim some of Hemingway’s words that such use was minimal and qualified as fair use of the material.”
From here