Umberto Pellizari the author of "The Manual of Freediving "is a pioneer of the sport. The original Italian text was translated to English by World Record holder freediver and spearfisherman , William Trubridge. They are currently working on a new version. The original is somewhat outdated but it is the first comprehensive publication on freediving. The sport has progressed greatly since its publication and there are many new theories and training programs.
Breathology is by Stig Severinsen , former record holder , scientist researcher. Here is his website :
Breatheology
Federico Manna has great books on breathing and equalization, also.
movinglimits.com
All of the books focus on relaxation and flexibility. That is why the yoga is in all of them. A flexible diaphragm is an important element to an increased breath hold. Relaxation will help you burn less oxygen; that is one reason your breath hold improves as you dive more. The more comfortable you are, the less oxygen you waste. Good fin and dive technique also make you more efficient.
Reading these books is helpful and then you need to go out and practice the techniques. Have a buddy film you with a GoPro and you can see what you are doing well and what needs work.
Hope that helps.
Carla