...We have read all the books and our marriage is better than ever, except BH wanting to send a letter to OM�s wife, telling her everything. Does she have a right to know?
Yes; but more to the point, you have no business disagreeing with your husband on this point.
From the standpoint of your OWN marriage: If you've "read all the books," then you understand that the purpose of exposure is to kill the affair. The 2 people to whom exposure is most likely to be effective are the 2 betrayed spouses. If only one of them knows, sometimes that's all it takes to kill an affair; but why should your husband bet on that? After all, he never bet on you falling for another guy, either. Your husband wants to expose because this will add an extra layer of assurance; the other man's wife's knowledge of the real situation will make it further less likely that the affair will resume. There'll be another pair of eyes on your OM. That's good for your marriage. And you're against this, uh, why?
...This will hurt her so much and they have little kids. Who is the best person to tell her?
Your husband is the best person to tell her. Yes, it'll hurt. BUT:
From the standpoint of THEIR marriage: Her NOT knowing will leave them set up for much more serious hurt later on. The TRUTH will give them a chance to look at their marriage for what it is -- i.e., vulnerable to an affair, with emotional needs not being met, and one of the people in that marriage having dangerously weak boundaries. This is just as important, I would argue, in cases of emotional affairs -- because most people make the mistake of failing to regard emotional affairs as seriously as physical affairs. I had both; one led seamlessly into the other, and as far as I'm concerned, the difference between an emotional affair & a physical affair is merely the difference between the top half and the bottom half of a hand grenade with the pin pulled out -- if you let them hang around & don't do anything, it'll all blow up & make a mess of your life. Her having the info about his emotional affair might enable them to confront their unmet needs & his lack of boundaries and right the ship and prevent what happens to marriages where needs go unmet & boundaries are loose. Yet you would deny that woman this information. That is not an honorable or defensible stance on your part.
...I just want to move forward but BH won�t let this one go.
No. You cannot move forward until your husband is ready to move forward. You move forward TOGETHER. THAT's how you restore a marriage & make it better than it was before the affair. If you've read the RIGHT books, you know that one of your key jobs in the recovery phase after an affair is to protect your husband, and not be the cause of his unhappiness. (Read about the "Rule of Protection" on this site if you're not clear on that.) I believe this is a situation where this applies. More basically, having the other man's wife know is an extra precaution against the danger of resumption of the affair. This also applies.
... I realize the books say to expose but what if there is no danger of the affair starting again?
Ma'am, who gets to judge whether there's "no danger" of the affair resuming? That's a pretty subjective question, isn't it, whether there's "no" danger? Isn't there "some" danger? And how much "some" should your husband be expected to tolerate? Well, that's up to him. Not you. If a betrayed spouse is trying to save a marriage, he/she has to be sure that the affair is dead. And when you're trying to make sure an affair is really dead, it's safer to fire every round in the chamber, so to speak, than to take one shot at a time & risk finding out later that the affair wasn't really dead, just wounded.
I encourage you to consider.