A selection of pdf files on yoga by other authors to download. I do not endorse everything in these books, but I have personally found them informative and illuminating and have come across them for free on the internet, so I thought it a good idea to organise them all on one site.
Books on Kundalini Yoga
Kundalini Tantra by Swami Satyananda Saraswati.
The Serpent Power (The Secrets of Tantric & Shakti Yoga) by Arthur Avalon.
The Kundalini Shakti (The Primal Power in Man) by Swami Narayanananda.
The Kundalini Experience by Lee Sannella.
Kundalini (The Evolutionary Energy in Man) by Gopi Krishna.
Kundalini - The Secret of Yoga by Gopi Krishna.
Devatma Shakti (Divine Power) by Swami Vishnu Tirtha.
Books from Early Theosophy
The Hidden Side of Things by Charles Leadbeater
Clairvoyance by Charles Leadbeater
The Chakras by Charles Leadbeater
How Theosophy Came to Me by Charles Leadbeater
Man Visible & Invisible by Charles Leadbeater
The Astral Plane by Charles Leadbeater
Thought Forms by Anne Besant & Charles Leadbeater
The Etheric Double by Arthur Powell
The Astral Body by Arthur Powell
The Mental Body by Arthur Powell
The Causal Body & the Ego by Arthur Powell
Astral Bodies or Doppelgangers by H P Blavatsky
Yogi Ramacharaka
Advanced Course in Yoga Philosophy