I just finished reading the Hatha Yoga Pradipika by Shailendra Sharma. It is a quick and easy read, but unfortunately lacks detail for many of the practices. However, when detail is provided it is of the sort that is hard to find elsewhere. As an example, here is the first shloka on nadi shodhana.
nadi shodhan
Sitting in Baddha Padmasana, the yogi should inhale through the left nostril, drink the vayu and hold the breath to full capacity, and then exhale through the right nostril. Then, inhaling through the right nostril, gradually drinking the vayu and retaining it, he should perform kumbhaka as before and then exhale completely through the left nostril.
Sit in Baddha Padmasana and do Khechari. Put your chin down. Now close your right nostril from inside the nasal cavity with the tongue and inhale from the left nostril. Then close your left nostril from inside with your tongue and exhale from the right nostril. When you are doing Baddha Padmasana it sort of squeezes your Sushumna, especially near Anahata Chakra. Without Baddha Padmasana and Khechari, Nadi Shodhan is not possible. It also opens your lungs and your abdomen in a certain way to start a very special reaction of intestines. Opening the lungs enables you to use its full capacity while breathing, so you can fill up your lungs as well as abdomen with vayu and are able to do Nadi Shodhan.
In the Hatha Yoga Pradipika by Swami Muktibodhananda of the Bihar School of Yoga, the sitting posture is translated as Baddha Padmasana, but then it is recommended to not even attempt to sit in Padmasana let alone Baddha Padmasana. Of course, his book is for the general public, while Shailendra's book is for "very senior yogis" only as he says in the Forward.
