William Walker Atkinson 
Series of Lessons in Gnani Yoga, The Yoga of Wisdom [EPUB ebook] 

The Yogi Philosophy may be divided into several great branches, or fields. What is known as Hatha Yoga" deals with the physical body and its control; its welfare; its health; its preservation; its laws, etc. What is known as ’Raja Yoga’ deals with the Mind; its control; its development; its unfoldment, etc. What is known as ’Bhakti Yoga’ deals with the Love of the Absolute-God. What is known as ’Gnani Yoga’ deals with the scientific and intellectual knowing of the great questions regarding Life and what lies back of Life-the Riddle of the Universe." So begins "A Series of Lessons in Gnani Yoga (The Yoga of Wisdom)" by Yogi Ramacharaka. (Goodreads)