Breathing exercises are a part of virtually every mystical society from the Sufis to the Yogis to the ancient Mystery Schools. Here are some spiritual links on breathing to explore:
:: Wahiduddin page on breathing.
:: The Sufi Message, Volume XIII, Pasi Anfas: Breath
:: The Sufi Message, Volume VII, the Mystery of Breath
:: Breathing for life. Sufi breathing techniques and more.
:: The Art of Yoga Breathing (Pranayama)
:: Hatha Yoga Breathing
:: Anatomy of breathing