This book is amazing. Two useful bits that really helped me were mastic gum and mouth tape. Both of them made it much easier for me to breath through my nose at night and avoid waking up with a dry mouth in the morning.
There are also some interesting bits on breathwork and the scientific aspects of it. I was able to use those techniques to temporarily lower my heart rate to 45 BPM during meditation.
I was surprised at how quickly my rhr came down after I started cycling more even though I've never been very active in my life.
It also (I think!) helped with my sleep apnea/general sleep problems, and I've always assumed a good bit of that was literally just being better at breathing.
There are also some interesting bits on breathwork and the scientific aspects of it. I was able to use those techniques to temporarily lower my heart rate to 45 BPM during meditation.