Yoga videos and books
First, there is no substitute for a live instructor with years of experience that can help you, personally, perfect your knowledge and practice of Yoga. That said, there are many videos and books available on the market that can help supplement your practice and enjoyment of Yoga. Your choice depends highly on your dedication, your experience and your intention for practicing yoga. 

Be aware that western practice and translations of Sanscript terms and asans are not completely accurate. Because of this, and the evolution of many personalized styles, you will find many moderations of any one posture or series of postures. 

For instance, the term "Yoga" is actually a variation of the original term "yog", as well as the word asana is "asan" in the original language. Also, the posture Tadasana (Tad= tree, asan= pose), is often taught in English as "Mountain" pose. Whereas the true "Mountain pose", perbut-asan, is taught as "Downward facing Dog". 

As for the postures, the western world has come to represent  "yog" in various moderated forms. To get a true sense of "yog" in the orginal state, one really must travel East, or find a Guru who has studied in the East under a true Yogacharya (Yoga Master). This is not to belittle or criticize Western Yoga, but to call awareness to the true nature of Yoga. Hopefully this awareness will awaken your interest enough to motivate you to learn the asanas in their perfect, unmoderated state. 

If you are just beginning, you will probably want a very simple book that shows a few of the simpler and most popular poses, that is illustrated with examples of each pose as well as an explanation of the spiritual, emotional and physical benefits of each pose. Check out the illustrations and read their explanations. Be sure they are poses you are willing and prepared to learn, and that the instructions are easy to understand and follow. If they are, read the introduction to get a better feel for the philosophy behind the book. Be sure the book you are getting is for learning yoga or whether it is for promoting yoga.

As I have found with most areas of instruction, it is best to acquire at least two or three different books. Each book will undoubtedly offer a different approach to many of the same poses, and some will offer completely different poses in order to round off your knowledge of basis yoga asanas.

As for videos, you might wish to choose your video based on the title depending on your desired goal. They are generally labeled for "Strength", "Flexibility", "Tranquility", etc, and according to the practitioners experience: beginner, intermediate and advanced. As I have mentioned, this does not always mean they offer the correct series for you. 

Most videos tend to be for those who have already begun learning yoga and wish to use them for their daily home practice. These videos throw you immediately into a series of poses giving basic instructions without the time to learn them unless you keep rewinding your tape. Owning several videos for home usage is recommended, but before you fill up you video library with videos, buy just a few and practice them regularly to get a better idea of what you personally need out of a yoga video. You can never do any particular series too often. 

Remember, each muscle group and as well as ligaments that surround a hinge joint has a counterpart. If you work one set of muscles, be sure to work its counterpart. If you work the abdominals, be sure to work the back. If you work the left side, be sure to give equal attention to the left side. It is not impotant to get every single muscle in your, but it is important to complete a series that involves balance. 

But as we mentioned, there is no substitute for a true Guru.