The quote that stands out in my head is "no functional adult should attempt to control the emotions of another functional adult", Ethical Slut
However, I also believe the only thing that makes any marriage work is 100% honesty. In my opinion this sounds more like an affair (even without the sex she withheld information, which to me means she didn't trust you fully).
I am not sure how to help you in this, but my advice is this. It is very important for us to be honest, that's what people say when the basis of a relationship is "communication" what they mean, I believe is honesty. I don't think she should have kept these letters a secret from you. I wouldn't do that with my wife or our partner (we are in a trinogamous relationship with a bi woman).
If i were you, I would have a very serious talk with her that moving forward you need to be honest 100% with each other. The commitment is that in that contract of honesty you have to also take responsibility for hurt, anger, jealousy, own that and not take that out on her. In our experience with poly relationships or swinger relationships the most VITAL element to maintain truth and honesty, be honest about your feelings, and just never try to tell someone their emotions are not valid. We can't control how we FEEL or become attached to others, we can control how we act based on that. Her actions were dishonest.
Buddha said clinging to anger is like gripping a hot coal with the intent to throw it at someone, you will get burned. I believe in forgiveness, but I also believe you can't forgive until someone realizes how their dishonesty hurt you and asks for forgiveness. Once you have worked past the dishonesty (which can be difficult) and move towards fierce commitment to truth, and open communication, then you can choose if monogamy is what you ARE as a couple (in other words, if she enters another relationship SHE is choosing him over you). If that is the reality you may lose her.
If you can see one of the most important truths in life I believe which is from the book "Ethical Slut" (a book that REALLY could help you A LOT), "Love is not a starvation economy". TIME is though, and if you need her time, she will have to figure out how to balance that, as far as love goes though, when you love someone you love them, and it's possible to love your wife, and your kids and your parents completely... it is also possible to share romantic love, and unlike starvation economies where it is divided, it can even grow stronger.
Our love for our partner boned my wife and I MORE not less, we are more thankful to each other, and the addition of a third has brought more love into this house. It's just a question of breaking out of the mold of "normal" societal imposed monogamy (which in evolutionary terms is actually not natural for any mammals including humans, and a relatively recent development).
I think if your wife means enough to you, if you truly LOVE her, rather than want to possess her, if you can get past the idea that you can only love one person, and get past the idea that love is not divided it just grows to accommodate then total honesty is the only hurdle. In fairness you should see how she would feel if put in your place. It is important for your health, security, etc... that you know she isn't just wanting to "try this guy on" before she leaves you. If it's truly poly, and it's a relationship she needs to see through, then opening up and letting go may be the only way to keep the love you have. It's a difficult situation you are in. I would strongly recommend you read "Open", and "Ethical Slut" both books will give you amazing insight and even coping mechanisms.