I believe, agreeing with YogaPrem, that one should prioritize to learn Yoga methods directly from a certified and experienced yoga teacher.
I also believe that, agreeing with CityMonk, the best way to invest in a book is to clarify your need and purpose.
For example, in case you are interested in knowing either the most commonly used Yoga methods or Hatha Yoga, etc., Asanas, Pranayamas, Mudra and Bandha by Swami Satyananda Saraswati, is the best book, However, if you interested in practicing either asanas and pranayamas, therapeutic Yoga or Iyenger Yoga, 'Light on Yoga' and Light on Pranayamas by B.K.S. Iyenger are the best. Books like "Yoga - Body, Mind and Sou"l by Swami Sivananda and Illustrated book of Yoga by Swami Vishnu Deva Nanda are the best for a person interested in practicing Hatha Yoga or Sivananda Style of Yoga.