Without movement in your spine, you have no yoga practice.  Vanda Scaravelli's playful approach to understanding the body and spine, in particular, is coupled with stunning photography.  In one photo she illustrates Yoga Nidrasana, and she is about 80 years old in the pic.  Easy to read, I quote from it all the time in my yoga classes.  Not your 'how to' yoga book, but great for intermediates and advanced yoga students, too.

“Why are we doing yoga?  For health reasons?  Perhaps a walk in the park would be better.  To help someone else?  There are so many ways of helping people…No, nothing of the kind.  No motivation, no aims, only an agreeable appointment for the body to look forward to.  We do it for the fun of it.  To twist, stretch, and move around, is pleasant and enjoyable, a body holiday.”