There are literally hundreds of great yoga books. I've listed a few below that I hope you will find helpful, but it's by no means exhaustive. Please remember that the best way to learn is with a teacher, so that they can help you to modify poses to meet your own needs and ability, adjust you where necessary, and emphasise safe practice - so it's best to practice what you have learnt in class and are confident with, mindfully. I hope you will find the following list a useful starting point.
You may also like to try Yoga Journal and Yoga International on-line for their home practice section.
For those doing teacher training / Foundation Course, also:
Teaching yoga, exploring the student-teacher relationship by Donna Farhi - a great book on ethics and the role of the teacher, with sections on how to handle a number of potentially tricky dilemmas or situations you may come across as a yoga teacher.
Your body, your yoga - Bernie Clark - an absolutely fantastic book on individual differences and how that affects yoga practice. 'Practice and all is coming' is not available to all students! This book explores 'what stops you' and helps us as teachers to understand where we can help students best. His follow up book Your Spine, your yoga, is also excellent.
Ray Long's books on Yoga anatomy and postures - great illustrations that show the anatomy within yoga poses - highlighting agonists, antagonists, stabilisers and breaking down the anatomy of yoga asana.
Very Short Introduction to: Indian Philosophy, Sue Hamilton – Oxford university press
Yoga Body - the origins of modern postural practice – Mark Singleton
Roots of Yoga – James Mallison and Mark Singleton – draws from a great variety of traditional texts – covers a wide range of concepts and practices. Detailed and thorough
The translations of the classic texts I used during my own teacher training were these:
The Yoga Sutras of Patanjali, translation and commentary by Sri Swami Satchidananda, Integral Yoga Publications, ISBN 0-932040-38-1
I can also highly recommend Edwin Bryant's edition of Patanjali's sutras which contains a number of different commentaries for contrast.
The Bhagavad Gita, translation by W J Johnson, Oxford University Press ISBN 0-19-283581-5
The Bhagavad Gita, a walkthrough for westerners, Jack Hawley, New World Library, ISBN 1-57731-147-
The Upanishads, translation by Patrick Olivelle, Oxford World Classics, Oxford University Press, ISBN 0-19-283576-9
Hatha Yoga Pradipika – Swami Muktibodhananda