Thank you 
My interests and style of practice have changed over the years but i think these are all really worth while, regardless of style of practice.
The Yoga Tradition (Georg Feuerstein) - covers everything you could ever need to know about yoga history and philosophy, including some of the original texts. Not a quick read.
Yoga: The Spirit and Practice of Moving Into Stillness (Erich Schiffmann) - my all time favourite yoga book. He is just so lovely and the style so gentle. He talks a lot about learning to work the edge, which transformed my practice.
How Yoga Works (Geshe Michael Roach) - odd book. It is written as a novel and while it is not great literature it does explain the Yoga Sutras of Patanjali in a very accessible way (i don't actually agree with one of the threads of the book, that the tradition should never be changed, but that's a whole new thread). I learnt a lot from it.
Asana, Pranayama, Mudra and Bandha (Swami Satyananda Saraswati) - probably the best book on practice, but also a really good read.
Yoga Body: The Origins of Modern Posture Practice (Mark Singleton) - illustrates how much nonsense there is about lineage and the origins of many asanas. Really surprising stuff.
Yoga Beyond Belief: Insights to Awaken and Deepen Your Practice (Ganga White) - i put off reading this for ages as it has a forward by Sting (not a fan!), but am glad i did - the approach completely resonated with me, and if you are interested in yoga philosophy as well as practice it is definitely worth a read.
There are many others i really like, but these are the ones that have really taught me something. Love the thread title btw!