I’m not sure about any literature in English, but you should definitely consider to buy a watch that measures these levels. I know a Norwegian doctor and author that has written a book that is called “the pulse cure” and have spoken to him regarding the HRV. I guess the book will be available in English soon. I use a Garmin watch to collect my levels. Ice bathing is also a insanely effective way to increase your HRV. I’m soon to be 40, but my biological age is now 32 years🥶🌞