One of the issues faced regularly by archives which hold the correspondence of a prominent person is that the letters will have multiple copyright owners. Copyright in a letter belongs to the author and typically there will be many authors. This becomes an issue when you need permission to digitise or publish letters from the archive.
The ownership of copyright in letters has been thrown into the spotlight by the legal action brought by Her Royal Highness the Duchess of Sussex against Associated Newspapers with regard to the alleged publication of private correspondence in the Sunday Mail. Infringement of copyright is one of the claims, perhaps the main claim.
The underlying copyright issues are discussed in depth here on the IPKat blog including the difficulty the newspaper might have if they try to claim their use of the letters was covered by one of the exceptions to copyright contained within Section 30 of the Copyright, Designs and Patents Act 1988 (CDPA), which deals with “fair dealing” for the purpose of criticism, review and quotation more generally. The difficulty for the publisher is that the letters in question must have been previously “made available to the public” with permission of the copyright owner (the Duchess in this case), otherwise the exception does not apply (by virtue of CDPA Sub Section 30,1ZA,a). It seems unlikely they could claim that this condition has been fulfilled.
Although copyright is often in the headlines with respect to the music business it is less common to find copyright in unpublished literary works featuring so prominently.