Cinco, I know it should seem like you only need to say something so drastically important once...and that it should sink in from there and not be an issue again. That would be logical. But...as you know by now...emotions are not logical. Your emotions and your wife's emotions are ruling this sitch, not your logic or her logic. Of course if SOMEONE (ie: you are going to have to do it) would snap their fingers in your wife's face and say "hey...remember? Filed for divorce once already? You agreed to read the book? Nothing has changed and therefore, good ol' Cinco was caught reading Divorce Busters website....because remember? Those ultimatum talks?" Then you wife will go "oh yes, logically it makes sense that since I haven't changed since those talks and I did not read the book I agreed to read, that he actually MAY divorce me. Makes perfect sense". But until those fingers get snapped, she is in a could of emotions that will not look toward logic on its own.
I hope it all works out for you because from everything you have described, your wife really does love you and does not want to lose you. She just doesn't really "get it" yet that she has to step up to the plate. Is she worth fighting for? I have to assume that she is, being that you have been fighting very hard to save this marriage so far.
I can tell you that if my ex-h had put in half the effort that you have, I would have taken a lot of notice...but on the other hand, if he let things die down again and didn't push me to improve, we still would have ended up divorced anyway. (But he never put in the effort to begin with so we never even made it as far as you have).
I am glad you are prepared to move on if it comes to that. I think everyone should always be prepared for that because if you aren't willing to even look at that option, it will inhibit your ability to make good choices.