There is a new book on ballroom called Irvine Legend. At this point it is not listed. It appears to be compiled from Irvin' notes after he passed away. We had a quick look through and it apears to be the best of its kind. In it it has the dancer with the bones of the feet up through the leg and to the spine and to the head showing where they should be on this particular move.
There is a very interesting comparison of Len Scrivener, the way it used to be, and William Pino the way it is done today on a Lunge to the Right. There is a big difference.