Might I also suggest, that should OM contact you again, tell your H about it BEFORE you make any decision to talk, e-mail, IM, the OM. If you are totally open and honest with your H beforehand and including him in the decision on what to do about the sitch, you are showing him that you are really committed to making this work. You will score BIG points in rebuilding trust by doing this, by showing your H that you care about and respect his feelings.
I just wanted to post that again so you would read it again. Let your H be a part of this. You can't control what OM does. You can control how you react. Be totally open and honest with your H.
I think you should tell him about the letter and ask him if he wants to read it. Men and women are different. I would definitely want to read it, then I would want to be present while H called OW and told her to never write him again. Your H may not want to read it, but you should tell him and discuss how to handle it.
It may or may not help him to read it, but he deserves to know about it. The lies are what makes everything so painful.