As one previous reviewer says, if there is one reason not to buy this book, it's because it may be disheartening for complete beginners to see the more advanced poses performed by BKS Iyengar at the peak of his abilities.
This is not a book that was designed to impress the reader, but produced in the true spirit of a guru to guide and encourage yoga students. Yes, the photographs are in black and white, but they are clear. (I particularly like the one with the sofa in the background; it demonstrates that it really is the content and not the style that is important, unlike many more modern yoga books.)
I've been studying BKS Iyengar's method of Patanjali's yoga for 11 years and teaching for 4; I always take Light on Yoga to classes and it often comes out of the bag to provide the answer to a question.
If you are just beginning your yoga journey, I'd also recommend BKS Iyengar's The Path to Holistic Health but start with Yoga Explained by Mira Mehta. This is a lovely book by one of Mr. Iyengar's most respected students and one of the UK's top teachers. A couple of steps along the path and you'll definitely need Light on Yoga; if you need some inspiration to encourage you, buy it now.