To be brutally honest - HER letters to him reveal nothing but what she says and what she thought or felt.
Now, if she has letters FROM the stain we are talking major scoopage.
Otherwise it is nothing but a he said/she said attempt to sell books.
Exactly right, IMHO. It would be a mistake to invest too much credibility in her salacious tale — unless, as you say, she also has “from” letters.
It’s unlikely she has any “from” letters, so long as B.J. followed the old dictum: “Wrong no man, write no woman”. (OTOH, he did stain a certain dress, so maybe he got careless with the pen too.)