The practice of Asana or Yoga Poses is only 1 aspect of yoga. Yoga is also a way of living and the philosophy of yoga is "practiced" as well.There are many great books out there on Yoga, but the books below form the cornerstone of any balanced practice.
The Yoga Sutras of Patanjali
The Yoga Sutras is hands down one of the most important books ever written about Yoga. Scholars believe that it was written somewhere around the second century BCE and it explains what Yoga is and how to incorporate it into your life. There are many interpretations of the ancient Sanskrit text but my favorite one is by Swami Satchidananda.
The Bhagavad Gita
The Gita is pretty scary to alot of people because it is a Hindu Holy Book. Amidst the stores of Gods and Goddesses there is a considerable amount of commentary on Yoga. It was composed somewhere between the 5th and 2nd century BCE and there are many translations for this one as well. It tells the story of Arjuna, a man who is torn because he is forced to go to war with his own family members and loved ones. Lord Krishna, a Hindu god, comes to him in the form of a charioteer, and gives him wisdom to help him in his anguished state. The one I read was, The Bhagavad Gita: A Walk Through For Westerners.
Light on Yoga by BKS Iyengar
Light on Yoga is one of the most comprehensive texts on yoga philosophy,Asana ( Physical Yoga Practice) and Pranayama. If you can only get one book that encompasses everything, this is the book for you. It includes the Yoga Sutras as well as detailed pictures of Asana poses and how to practice them. It doesn't replace a good teacher but it is a great supplement.
The Hatha Yoga Pradapika
It is the oldest surviving text on yoga. I have yet to find a translation I like though. Maybe you can suggest one to me.
The Heart of Yoga by TKV Desikachar
This book was written by the son of Krishnamacharya. Krishnamacharya gave us the gift of Iyengar, Ashtanga, and Vini Yoga and his teachings pretty much formed the foundation of most westernized systems of yoga. This is a beautiful book that seeks to shed light on the man who revolutionized yoga and set the record straight on his teachings. It also teaches you about forming a personal practice.
Asanas: 608 Yoga Poses by Dharma Mitra
Dharma Mitra took pictures of himself practicing as a gift to his teacher and the shots were later made into a book. It is a beautiful collection of poses that will challenge any student.