Yes, letters have been a big help. You have to really stop and think about what you want to say. There are drawbacks to doing it that way, too, I'm finding.
What seems to be working is that if something comes up that one of us feels a burning need to talk about, we can say, "can we schedule sometime to talk about such and such?". That way, no one is getting blindsided or put on the spot, and things aren't always discussed in the heat of emotion.
I have told H several times over the past few months that I had a lot of questions to ask about the OW and the affair. I didn't want extreme detail, but needed to hear his story. Last night, we were able to start that process when I asked H to start telling me, given what he's learned in Counseling, what he would have done differently at different steps. He was able to give me some good information that lets me know he's done a lot of thinking about how NOT to get in that situation. I didn't push too much, but was able to ask some questions and let him know that I wanted to continue talking about that subject as we go along.
I had been a bit afraid of beginning to really talk about the OW. I was afraid he'd get really defensive, and that I wouldn't be able to help but get angry. Framing it the way we did made things feel safer for both of us.