Are you interested in yoga or are you interested in asana?
If you are interested in Yoga you'll need a book on living. If you are interested in asana then a book on poses and postures.
It might be helpful to begin by browsing the Shambhala Encyclopedia of Yoga authored by Georg Feuerstein. It is a comprehensive glossary of terms that will be plucked off your shelf many times over.
My teacher, Aadil Palkhivala, has also mindfully written a book. It is called Fire of Love: for Students of Life, for Teachers of Yoga. There is not one posture in the entire book as it is all about living. It wil only change your life. Your hamstrings may remain the same.
If you fancy going into the Yoga Sutras and doing so a bit deeper than the average bear who would likely pick up the Swami Satchidananda version, try The Science of Yoga by I.K. Taimni. This is one of the most probing, insightful, discourse on the sutras I've ever seen.
Asana books abound. And while Light on Yoga is the definitive text on asana it is a bit dangerous in the hands of a beginner. It wold be difficult to discern what is for you now and what is not when you have the entirety of asana in one collection.
Pranayama or the direction of breath should be learned from a teacher please. Once again, it would be difficult to discern what is and is not for you without some guidance and book guidance is often both indirect and misdirected. If you must start on your own - and I understand that "must" - please begin with gentle pranayama and do not advance until you've mastered them over the course of several years, if not longer.