The following books are primary sources and recommended reading for practice within the Iyengar tradition.
These books, along with a wide range of yoga props, are available through iyogaprops.
The images below link to either PDF versions or further resources.
Further readings of interest:
Why does anyone do yoga, anyway? by Marlynn Wei.
Krishnamacharya, The Man Behind Modern Yoga by Azriel ReShel.
Looking beyond the lens of Instagram perfection in Yoga by Heather Mason.
BKS Iyengar: The man who helped bring yoga to the west – BBC 21 Aug 2014