I undertsand she is selling letters from George Harrison and Eric Clapton.
The letters are still subject to copyright. The author of the letter is normally the first owner of copyright in their literary work. Copyright lasts for the life of the author plus 70 years.
The recipient is the owner of the physical object.
Where the owner of the letter is not the copyright owner, they must first seek permission from the copyright owner before reproducing the letter. However, copyright does not prevent the recipient of a letter from showing the original letter to another third party as this does not involve reproduction.
This would prevent Christie's reproducing any of the content in its sales catalogue or any buyer from, say, reproducing, the content of the letters in a book without the express permission of (presumably) Olivia Harrison or Eric Clapton.