I'm reading a great new book by Peter Wayne called The Harvard Medical School Guide to Tai Chi: 12 Weeks to a Healthy Body, Strong Heart, and Sharp Mind. The author is a researcher at Harvard Medical School and also has been a tai chi instructor for 35 years.
Great book! I would highly recommend it if you are doing tai chi or if you are interested in it for health reasons.
I learned it years ago in a course at our hospital. It's wonderful for good health, breathing and relieving stress. I also found that it was very good for balance in my later years. In the class we did the motions for an hour. In my busy life style I liked to cut it down and not do as many of each one. Probably not as beneficial but better than not doing it at all.