William Walker Atkinson 
Series of Lessons in Raja Yoga 

One of the greatest books I have ever read. It explains the processes of the mind on a level so clear that anyone should be able to understand the basic concepts which lie within the great depth that is our own mind (and you will find if you are well trained that it is the same thing as the mind of others!). Highly advanced meditation practices are included, and transcendental states of mind are within reach for anyone who chooses to study using this book. Psychedelic users can in effect experience highly similar states of mind without the use of any substances via the methods in this book, though it will take serious concentration and a highly supportive environment. (Goodreads)