The one Ramsay sends to Jon about several different things depending on if you're in the book or the show.
I don't think it was more of a focus in the book necessarily. It was part of the gigantic shit-hits-the-fan moment at the Wall after loads of build up, and it has potential to be more focal in the future, but we didn't get much time with it in aDwD
I do like the book theory that
Ramsay didn't write the letter though. I kind of doubt they'd have that kind of complexity in the show at this point - the shoe fits so Ramsay will just wear it and it will work.
Hey by the way anyone remember Dorne?