I came across Cheng Hsin: the principles of effortless power by Peter Ralston in 1990. This is my first copy and is a bit battered now.
It is full of useful stuff about posture, alignment etc but what is most interesting and resonated strongly with me is the discussion about relationships.
All partner work in Tai chi is a study in relationships and is a life long pursuit!
If you haven't read it, get it!