The information about shoulderstand comes from the following sources:
Light on Yoga by B.K.S. Iyengar, and Yoga, a Gem for Women by Geeta Iyengar both provide a list of shoulderstand’s potential benefits and counterdications based on their experience with thousands of students over the course of over half a century of teaching.
Anatomy of Hatha Yoga by David Coulter gives an excellent explanation of the features of shouolderstand that make it difficult to do. It also provides a methodical explanation of the pose’s beginning and advanced variations.
A Handbook for Yogasana Teachers, by Mel Robbins contains a rich explanation of the possible physiological effects that can make shoulderstand (after much practice) such a relaxing pose.
The YogaWorks teacher training, provides a list of shoulderstand’s benefits and counterindications which, in keeping with the rest of their training, is at the same time more succinct and more comprehensive than almost any other source I’ve found.
Barbara Benagh has brilliantly explained the benefits of working up to shoulderstand incrementally through its many variations, in her shoulderstanding workshops.
Thanks to Peentz Dubble, one of the best Iyengar instructors in New England, for encouraging me years ago to incorporate shoulderstand into every asana practice session I do. It has paid off richly.