Xing Yi Nei Gong is a terrific book translated by Tim Cartmell and Dan Miller.
Although this deals with XingYi the exercises are great even if you don't study it. The book has a lot of information and the section on Standing Practice is just as applicable to Tai Chi.
You can get a DVD of the exercises but I didn't think it was very good.
I don't know if this book is still in print. It was published by High View Publications - ISBN 1-883175-04-6