Okay. This is going to take all your strength, but the very best thing you can now do is to let him go. Going to Relate now won't necessarily help - and neither will him reading books. This affair was evidently interupted and so if he thought his feelings were strong for the OW last week, they are not going to change overnight. However, it will tear you apart to see him grieving and nursing his loss and there is nothing like living the reality of a separated parent, to stop ambivalence in its tracks.
You might feel tempted to continue to live with him and wait this out until he is ready to come back to his marriage, but it will in fact elongate the ambivalence and destroy you in the process. Think of this now as ripping off a plaster - it will hurt like the devil right now - but it is by far the best strategy for you personally and very possibly, your marriage.
Tell me how long he admits that this affair has been going on and tell me whether she is prepared to leave her H for him, or whether he wants her to. Also, does her H know now about the affair?
These facts are key to the advice we can give you. If he is only willing to stay with you because she is not prepared/ready to leave, then don't let yourself be the soft landing. Feeling like he does, it is also very likely that their affair will continue in some guise, especially if they work together.
I would also reassure you that although at the moment he might think his feelings are very strong and that he is in love, this might be a temporary thing that will actually flounder in the face of reality and living with the consequences.
For you personally, do buy a book called Not Just Friends because it will really help you more than any other book, to understand what has happened here and why. It will help you to understand that it is no reflection on you or even your marriage, but that what was once a safe friendship evolved over time and that bit by bit, the boundaries around the friendship became blurred.
The strongest advice I can give you is to muster all your dignity together here and play the long game. Tell him to go and if possible, live on his own for a while.