I want to thank everyone who responded. This was my first thread and it was overwhelming, everyone who cared enough to help. When something like this happens, it really helps to talk it through and get feedback from those outside the situation. As I mentioned, I have told only one close friend about this, and that, only because she is in a position to observe first hand. I didn't want to blab to friends or family and do damage that could not be taken back.
My husband is a good man, the last person in the world, anyone who knows him, would think to be found in this situation. People respect and think highly of his character. I have always been proud of him and he has been close to and loved by my family (even the JW's). I don't want to rush ahead out of hurt feelings and destroy that. So, for now, I am going to continue to say nothing and act as if all is normal. I honestly do not believe it has become physical. From what my friend tells me of this other person, it will indeed run its course after she gets what she is after. Unfortunately, I have found evidence that it may be of a financial nature -- and that really ticks me off, but, oh well. My friend was completely taken aback about the idea of my husband, but not the woman. Apparently he is not the only male she has charmed to her advantage.
In the meantime, I will continue to be alert to certain things so that if it does indeed come to a showdown or has progressed to the point where it can no longer be tolerated, I will have given him enough rope to hang himself. From many of your comments, I have seen where I need to improve, where I have let the relationship grow stale and routine. I have to be the best I can be to work on his conscience and make him feel guilty everytime he does something that involves her. I also feel a boatload better, having found out that after the number of years invested in our marriage, I am legally entitled to half his retirement and social security!!! I will be upbeat and positive--a mopey, surly personality is attractive to no one. I will strive to withhold sarcasm...that will be the hardest part.
I will keep you updated. I have a feeling that if something major happens it will be in the next few weeks (when the bills come in). Thank you so much for your comments--except for one "lame"comment that was hurtful, you have all been appreciated.