The Science Of Yoga by I. K. Taimni - A commentary on Yoga Sutras of Patanjali. This might fall more on the religious side for many people but the author as well as the original scripture has nothing to with Gods in Hindu religion and describes how we perceive our reality and how should we proceed to understand it and ourselves further. I have read a lot religious scripture commentaries but this original scripture and author's interpretation of it is something next level for me.