If you are interested in the techniques of viniyoga and want to read more, I have found that Yogamatters carry an extensive range.

The web address is http://yogamatters.com/acatalog/Desikachar.html [1]

They also have the two DVDs that Gary Kraftsow has recently produced on back problems.

They can be found at http://yogamatters.com/acatalog/Gary_Kraftsow.html [2]

I have just ordered them for myself, and will post a review in the book section once they arrive and I have time to look at them.