This is a good and challenging question!
If I have to choose one:
Bhagavad Gita because it has everything contained in one volume.
Others at a close second would be:
Bliss Divine, Yoga Vasishta, Viveka Chudamani, Thought Power.
The book that first got me interested in yoga was ABC of Yoga by Kareen Zebroff - I grabbed the odd spiral-bound book off the shelf in my high-school library and for some unexplainable reason was drawn to the images so much that I knew I wanted to take it home and try it out.
A bit later, I found the Sivananda Companion to Yoga (originally in French and then in English) and that book became the foundation of my practice.
Finally, Le Hatha Yoga by Eva Ruchpaul is another book I like to look up occasionally because of its interesting insight on being a yogi and the beautifully inspiring pictures as well.
How about you Sukadev? If you got the ball rolling, you must have your own favourites....
In the Bhakti Marga, Vaishnava, the first is the 'Bhagavad Gita'. I love 'Teachings of Lord Kapila, the Son of Devahuti', too.
In Hatha Yoga, my first book, actually a booklet, was one called Selection of Exercises by Swami Pranavananda Saraswati. Then it came 'Forever Young, forever healthy' by Mataji Indra Devi, 'Hatha Yoga for Woman' by Zita Rajhavara, 'Cultura Física y Judo' by Dr. Serge Raynaud de la Ferrière, and so on.
There are other books, unforgettable books one can mention from Sivananda, Yogananda, and other yoguis, but in order to speak in short, the philosophical 'Bhagavad Gita' is the most inspiring for me.
Good question indeed. Bhagavad Gita is the book to be cherished and studied. The Yoga Sutra of Pantajali and for Hatha Yoga Swami Satyananda Saraswati Asana, Pranayama and Mudra Bandha book, it is a basic book with alote of information.
I can not stop to one book, but many...as the other members mentioned before, Bhagavad Gita, Patanjali's Yoga Sutra, and Raja Yoga by Vivekananda. I like many of Vivekananda's writings and not at least the Sivananda's collection.
For me, my favorite Yoga Book is my Sadgurudev Dr. Narayna Dutt Srimali (Paramhamsa Swami Nikhileshworananda Maharaj) Today I am Kriyananda Maharshi Dayananda and it is only by the bless of him. I read only some of his book, like Meditation, and others.
Although, I have not met him physically but took a Dickcha Sansar from Trimurty Sadgurudev (Sri Nandakishore Srimali, Sri Kailash Chandra Srimali and Sri Arvinda Srimali.) they are son of Dr. Narayan Dutt Srimali, yet I got that spiritual guidance and he made me Kriyayogi. Can you imagine how it could be possible but it is true.
My every thing is Sadgurudev and I have a desire to work for him as Gurusewa and that could be in different way.
So book can only give you reference, authors feeling and experience but book cannot teach you itself at all.
A. The spiritual journey tends to be a long one spread over numerous births/lives including the present one during the time which the true spiritual seeker learns and assimilates spiritual knowledge from numerous sources which may be people, books or some other experiences. So, it's really difficult to credit any single ENTITY for my spiritual progress. It just happened. On the way, I was blessed enough to learn from diverse sources and move on....DIVINE MOTHER HAS BEEN GUIDING ME ALL THE WHILE EVEN WHEN I HAD NOT BEEN BELIEVING IN HER/NOT FOLLOWING HER....SHE HAS BEEN MY BE ALL......ULTIMATELY, NOTHING CAN SUBSTITUTE ONE'S OWN INDIVIDUAL EXPERIENCE. IT's NO USE OTHER'S SPLENDID SPIRITUAL EXPERIENCE IF WE OURSELVES HAVE NOT EXPERIENCED IT...THIS IS THE CRUX/BASIC AXIOM OF THE SPIRITUAL JOURNEY....!.
B.Following are some of the USEFUL BOOKS that I have read personally and found as a YOGA PRACTITIONER extremely useful:
1. Hathayoga Prdeepekaa, Hindi language,Edited by Dr. Chamanlal Gautam, 2005 Edition, Sanskrte Sansthaana, Khwaazaakutub [Vednagar], Baraeli -243003 [ Uttar Pradesh].Phone: 474242
2. Sheva Sanheta, Hindi language,Edited by Dr. Chamanlal Gautam, 2005 Edition, Sanskrte Sansthaana, Khwaazaakutub [Vednagar], Baraeli -243003 [ Uttar Pradesh].Phone: 474242
3. Asalee Praacheena Sheva Svrodaya, Edited by Shivbhakt Baba Anurag Dass Jee,Hindi Language,2004 Edition, ISBN - 81-86955-86-0,Randhir Prakashan, Hardwar [India]
4. Vevaeka Chuuddaamanne, by Aade Shankraachaarya, Hindi Language,Code Number 0133, Gita Press, Gorakhpur, India
5. Upaneshada Anka, Hindi Language,8th Edition, Code Number 0659,ISBN 81-293-0166-0,Gita Press, Gorakhpur, India
6. Yogaanka, Hindi Language,8th Edition, Code Number 0616, ISBN 81-293-0048-6, Gita Press, Gorakhpur, India
7. Yogavaasheshtha, Hindi Language, 16th Edition,Code Number 0574, ISBN 81-203-0158-X,Gita Press, Gorakhpur, India
8. Daevee Puraanna, Hindi Language, Code Number 1610,Gita Press, Gorakhpur, India
9. Aarogya Anka, Hindi Language Code Number 1592,Gita Press, Gorakhpur, India
10. Shakte Anka, Hindi Language, Code Number 0041,Gita Press, Gorakhpur, India
11. Paatanjalyayogaprdeepa, Hindi Language, Code Number 0047,Gita Press, Gorakhpur, India
12. Gheranda Samhita, English Language,by Rai Bahdur Srisa Chandra Vasu, 1979 Edition, First Published in 1914-15, Sri Satguru Publications, 40/5, Shakti Nagar, New Delhi – 11007 [India]
( This book is available free of cost for Download on www.scribd.com ) ..RSS
Note: Some of the PURAANNAs and UPANESHADAs published by GITA PRESS GORAKHPUR really contain EXTREMELY useful reading material on YOGA and AYURVEDA/Aayurvaeda.
There is a very nice and meaningful quote and that is very self explanatory.
"Pothi padh padh jaga mua pandit bhaya na koi, Dai aakhar prem ka padhey so pandit hoy."
but please make it clear that I do not meaning that reading a book is not good but the truth is that you can only take the reference only and if you want to get the experience of the said book then you will have to follows according to its explanation but to guide the explanation in a right way you must needs such person who can guide you.
1. ANYONE who LOVES quoting KABIR must ALWAYS REMAIN aware of KABIR's FAMOUS SAYING - "DHAAE AKSHARA PRAEMA KAE [ 2 and a half WORDS of LOVE]" is the SIGN of a TRUE spiritual seeker. Anything done with a Divine feeling is the REAL YOGA - as is said. So, let us learn to accept the viewpoints of others with the Divine humility and Grace of a TRUE ASPIRING YOGEENEE/YOGEE...and not feel hurt.....!
2. Who is to decide as to who is to interpret the TEXTS? Anyone with a critical bent of mind can interpret these texts rationally in the light of the EXPERIENCES....Even in one PURAANNA, it is clearly mentioned that - "No Two reshes [spiritual scientists] are reshes if their viewpoints match with each other." So, the SCOPE for diverse interpretations in the light of the EXPERIENCES [Spiritual] has already been provided in our ancient WISE texts.....! What may be your spiritual experience may not NECESSARILY be the same experience with other seeker. The other seeker may well have different quite VALID experiences...!
3. So, all I can do is just SMILE...:-) at the DIVERSE views and different emotions springing out from the minds of different Spiritual seekers......May the Divine Mother give all spiritual seekers the EXCELLENT ability to interpret scriptures properly.....:-)