I think it's important to start by separating two things here: resonance breathing as a practice, and this book by Leah Lagos. Resonance breathing is something I would definitely recommend. I've derived a lot of benefit from doing it for 4 months or so now. I've been significant improvements in my HRV. I feel sharper and calmer. It definitely helps to wind me down for good sleep in the evening. This book, on the other hand, I would not recommend. You don't need it to practice resonance breathing, and I would recommend not trudging through all the other exercises unnecessarily.
My first gripe with the book is that it's poorly written. The language is dry and uninteresting, except when there are random insertions of contemporary colloquialisms that feel like they were added as an afterthought to make it more 'fun' and fall quite flat.
My second gripe with this book is that, while the core practice of resonance breathing and the guidance on how to do it are quite valuable, I found most of the other exercises unhelpful, poorly described, hard to execute and probably superfluous. It felt as if they were added just to build a 10 week programme. I'll give you a simple 10 week programme: breathe for 20 minutes, twice per day at a pace of 4 seconds in and 6 seconds out, for 10 weeks. I'm pretty sure that will get you at least 90% of the benefits.
My third gripe with the book is that it pretends to offer something that it doesn't really. It's packaged as if it's meant to be about HRV bio feedback, but then Lagos discourages you from using live biofeedback. I understand that this was probably to make the book more accessible to those who don't have the tools, but it does mean that the descriptions of this book are not really representative of the reality. It's not actually about HRV biofeedback; it's about improving your HRV through paced breathing.
My last complaint is the packaging as a 10-week programme. The reader is genuinely led to believe that something significant will be achieved in 10 weeks which will then last. I suspected this wasn't true from the start, and then at the bend of the book, Lagos confirms this. She states outright that to maintain all the benefits, you have to continue the practice at least 5 days per week. That's not surprising. If there were really practices that you could do like this for 10 weeks and the reap the benefits for life, everyone would be doing them. Life is harder than that; that's fine. I just don't understand why Lagos tries to suggest that this is simply a 10 week programme. It's a practice that you can choose to integrate into your life for a chapter or forever.
So, in summary, don't read this book. Take the 3-5 hours that you would have used to read it and do a week of 2x20 minutes of resonance breathing per day. You'll probably find the motivation to continue, and I'd suggest you do. I think it's a valuable practice.