Book: Control Of The Astral Body by Aleister Crowley
This matter of switching on the Astral Body is a personal trick. It's a straight forward exercise in imaginative Projection to visualize the thing in the first place. The main work of building it up and using it is approached by Crowley in his essays on Yoga, in Liber NU, Liber E, through a number of the rituals and in Some Other Libers. Crowley's Magick in Theory and Practice contains a good deal of material on astral bodies, methods of working and the like. To a limited extent, the terms "astral body", "body of light", "diamond body", "subtle body", etc. signify the same thing; but these terms also refer to different theories, practical approaches and gradations of success.Books in PDF format to read:
Michael Smith - Ways Of The AsatruAleister Crowley - The Heart Of The Master
Aleister Crowley - Control Of The Astral Body