What is "Mom's Letter" and should I sell it?
Solution 1:
It doesn't do anything, it is the character's mom last letter to him ( her death wish ). It doesn't matter if you sell it or keep it.
I don't want to spoil anything, but at the end of the game you will read it, so i suggest to keep it since it isnt worth almost anything anyway
Solution 2:
You can read it before the end of the game (anytime, not just at the end of the game).
All you have to do is go to where you sell your items in your loot bag, and go to "Mom's Note" and it tells you what it says at the bottom left of the screen.