>>1Cool recommendations. To add to your thing about self-hypnosis Dave Elman's "Hypnotherapy" is a great introduction to hypnosis for anyone interested. Guy made it for the medical world but it's just a really good introduction to what hypnosis is, how it works, and simple techniques for anyone interested.
In regards to belly breathing and progressive muscle relaxation I'd recommend Robert Bruce's book "Energy Work". Has lots of exercises on working with your body's energy and can boost any spiritual practices or your body's general health by a good margin. Quick, simple exercises and you can just read a bit each day to try and implement exercises. You'd probably want to use it to aid any breathwork you do.
Regarding breathwork you could try "Science of Breath" by yogi Ramacharaka
Easy read that takes an afternoon, talks about full breaths and the various, poorer ways people may breathe.
https://sacred-texts.com/eso/sob/index.htm