My H and I are voracious readers, but tbh neither of us had ever bought a self-help book, let alone read one and recommended it as much as we do this one.
On first reading, none of the "stories" seemed to match ours, but I could identify so much with its holistic approach to infidelity and why it happens. As it turned out, we had followed much of the advice in it before we read it, but with hindsight, there were still so many lessons to learn.
When my H read it the first time, like me because he couldn't relate to the stories, he didn't find it as helpful and I think he would have preferred a UK book, but he persevered with it and we found that the more we talked about the issues, the more it seemed to make sense. For both of us, the second reading provided the most insight and I think that was because we had reached a point where all the truth had emerged. By that time, we had also read some other stuff too and this book then stood out as the best of them all. Some of the books we read were terrible (the Relate one for example) and neither of us rated the Andrew Marshall book either, although that had good points.
After that second reading, my H started to refer to NJF as "the bible" of recovering from infidelity and now we have both read it three times, learning something new with each reading.
When he was struggling with his first reading of it, I recall we had a showdown when I said that it was incongruous that I was doing all the research into his behaviour, which was perhaps unfair as he had been through a long and helpful solo counselling programme, which helped tremendously. I remember feeling very strongly about this though at the time and consequently, my H realised pretty sharpish that persevering with the book was going to be essential.
It was quite a turning point, because he then started to do his own research (lots online), persevered with the book, ordered others and started driving the process of understanding. It seemed like a watershed moment when he said that he'd realised that it was actually more important that he understood this (and himself) than me. It was around this time that he started saying that he couldn't ever do this to himself again, let alone me.
By the time we reached this point, we were about a year on from discovery.
Hence, I would say to you that you need to raise your expectations of your Hs. My H in his former guise would have rather pulled his own teeth than read what he (and I tbh) would have described as a "navel-gazing" book, although like you both, I had always been much more comfortable with having conversations about relationships and confronting problems. Not reading the book is therefore a symptom of the bigger change that is necessary in your Hs and for me, my H needed to become the sort of man who was comfortable with difficult conversations and reading material that was painful and uncomfortable. There came a point when I was quite clear that I wouldn't stay with him if he didn't make those changes - and he knew I wasn't bluffing.
The changes in my H are remarkable and significantly, his relationships with others have changed as a consequence. He is now sought out by other men who are having difficulties in their relationships and is regarded as someone who has great emotional intelligence. He works in a male-dominated industry and talking about feelings and emotion is not common-place. He said recently that he likes himself much better now and if I'm honest, we have both been surprised by the collateral benefits of the new him - literally all his valuable relationships have improved. He is a better Father, son, brother, in-law, friend, manager and above all, a far better partner in every way.
So I think this all becomes part of your new "conditions" and the new relationship you insist on. I sometimes think posters on here think my H was always like he is now, but of course he was very different and those behaviours permitted infidelity and were precisely the ones that had to change. These were the insights that we had (him too) before reading Shirley Glass. He was in fact the first to identify that his selfishness and resistance to having "difficult conversations" had brought him to that point and this demonstrates a point I often make on here. Infidelity never really happens in a vacuum; there are always behaviours evident (and tolerated) that lead to that low point.
Refusing to read a book that would really help and given what I've said above, not persevering with it when it gets too difficult, is a symbol of what else needs to change, really. Your Hs need to drive this process, because this was all about them. The relationship changes necessary after an affair are often the easiest part of the process - it's the other stuff that's far harder; the lifestyle changes, the challenges to the way you have both been socially conditioned and the hardest bit of all, the changes the unfaithful person needs to make to his own character and attitudes - are the ones that really make the difference between survival and defeat - and I include amongst the "defeats" the marriages that survive on paper, but are characterised by bitterness and hurt/anger that just won't go away. Marriages that never really reach their potential.
Hence I wouldn't be discouraged if your Hs are initially reluctant to read this book, but do see the bigger picture here about what that reluctance signifies - this is not just about failure to read a book. It is about a failure to be introspective and self-challenging and feeling the necessary discomfort that heralds important change. Don't be afraid of holding out for what you need to happen - and remember, real and lasting change happens when it is driven by your Hs and not you. And if you don't see that change happening, never be afraid to review your initial decisions about staying together and making new conditions in your relationships.