This is a great book by Arieh Lev Breslow that will be of interest to Tai Chi practitioners, especially those doing Cheng Man Ching style and variations thereof.
The author studied with Ben Lo in the USA before eventually opening a Tai Chi school in Jerusalem. The book covers many aspects of Tai chi, from posture to rooting to fa jing to pushing hands.
There is some sage advice about a straight coccyx and how many misinterpret this to mean they tuck the pelvis, thereby creating an unnatural straightening of the spine. Instead the lower spine should be allowed to relax and hang freely without tension.
The only website I've found that offer this is: http://www.abebooks.com/book-search/author/breslow-arieh-lev/