The Man in Black's Letter was a letter that Randall Flagg gave to Nort to give to Allie. It lets Allie know that speaking the number "nineteen" will open Nort's mind to what lies beyond death, and he will tell her.
The note was marked with Allie's name in fancy cursive, which was used for the rest of the letter. After Walter's signature he also leaves a smiley face beside it.
The note itself reads:
You want to know about DeathI left him a word. That word is
If you say it to him his mind will
be opened. He will tell you what lies
beyond. He will tell you what he saw.
The word is Nineteen.
Knowing will drive you mad. But
sooner or later you will ask. You
won't be able to help yourself.
Have a nice day!
P.S. The word is Nineteen.
You will try to forget but sooner or
later it will come out of your
mouth like vomit.