This recent thread had a lot of information/links for body turn on the forehand.
Tennis Oxygen Youtube series.
Macci video on the forehand with body turn pointers-
An excellent book spelling out the biomechanical principles for the body turn is -
Technique Development in Tennis Stroke Development
, B. Elliott, M. Reid, M, Crespo. Available from the ITF for about $25.
I have recently been putting more body turn into a few returns and it works right away for more pace. As of now, its difficult for me to time under pressure in doubles - but I'm a believer.