If you are after a place to begin, Anne Cushman and Jerry Jones’s ‘From Here to Nirvana’ is an excellent resource.
This book gives you an overview of:
Indian philosophies and religions, including Vedanta, Yoga, Tantra, Hinduism, Jainism, Sikhism and so on.
Then, it gives summary information on about a hundred different schools, gurus, yogis, ashrams and temples in India. When compiling research for this book, the authors approached the gurus, schools etc personally for contributions and verification. So what you get is a pretty faithful representation of each school/method of teaching. The authors allow the reader to assess the information given on their own.
Helpfully, you get the run down on the differences between different styles of yoga: Ashtanga, Kundalini, Iyengar, Hatha, etc. I believe this is what you’re after.
This book offers a wonderful map of the whole Indian spiritual terrain which can often be confusing. The teachings and approaches differ dramatically from each other and this book captures and summarises the differences quickly and well. I’ve found it both helpful and interesting.
This guide is intended to be a practical tool for those bound for India on a spiritual quest. So it also gives you the run down on practicalities. How to get to X ashram, how do you apply, what the fees are, what a typical day involves. As an aside, while it was published in 1998, I’ve found it amazing that the processes and fees remain accurate in 2010.
And in case I haven’t praised it enough, it’s even got recommended readings so you can explore whatever takes you in a more in-depth manner, be it Iyengar yoga or sadhus.
Plunging into the Bhagavad Gita or The Upanishads is diving into the deep-end which can be overwhelming for most without any background. This book gives context.
Good luck
Y