The Tao of Natural Breathing by Denis Lewis An excellent exposition on the natural breath
Prana and pranayama by Bihar school of yoga
available online at https://www.pdfdrive.com/prana-and-pranayama-e33215204.html
Light on Pranayama, by BKS Iyengar, View online here
The Breathing Book: Vitality and Good Health Through Essential Breath Work by Donna Farhi
An accessible guide to breathing including a very useful section on unhealthy styles of breathing
Breath, Mind and Consciousness by Harish Johari Insight into Swara yoga (the art of balance)
Breath by Breath, by Larry Rosenberg A comprehensive study of the Anapasati Sutta
Breathe! You Are Alive: Sutra on the Full Awareness of Breathing by Thich Nhat Hanh
A excellent translation of the Anapasati Sutta with commmentaries
Yoga Rahasya by Sri Nathamuni
Very useful book by Krishnamacharya. Valuable advice on asana and pranayama practice
Yoga Yajnavalkya, translation by A. G. Mohan
Useful classic, offering a different perspective on the 8 limbs path and, 4 chapters on pranayama practice.
Moving into Meditation: A 12-week Mindfulness Program for Yoga Practitioners by Anne Cushman An excellent and practical guide to integrating yoga and mindfulness