There are many fine books available about practising yoga. It might be very confusing which one is best for your needs as you start out. I might suggest you rent a video/dvd from a library if possible to begin practising with some guidance. Books are a little challenging to learn yoga from as it is an experiential art and science and often requires the guidance of a qualified teacher or at minimum, a good video. There are many quality videos produced by Gaiam with qualified teachers such as Rodney Yee, Rod Stryker, Patricia Walden and others. I think you will find the actual guidance the best place to get started and then the books will serve as follow and for deepening your knowledge.