Just to add a bit to the topic: Iyengar has written excellent books on yoga and specifically pranayama. They are technical and detailed, with photos, and list the effects of every asana. His way of practicing yoga is different from Ashtanga, his asanas are static and pranayama is advised to be practiced under supervision and only after you have covered the basic asanas, whereas as far as I know in Ashtanga special breathing technique is incorporated from the beginning.
Iyengar's "Light on Yoga" can be a good start. I think it is available for a download somewhere on the web. There is also a chapter on pranayama in this book. It must be available for download on the web. If you were not able to find it..well,PM me
