I was trained to use the models outlined in this excellent publication 30 years ago, and have used them daily ever since. Having spent a lifetime in HR, Development and Learning I feel qualified to say that this is the best approach you will ever find to working as a consultant, leading and facilitating change and acting without the use of power and authority. The approaches outlined in the book are closely based on the work of Karl Rogers, the pioneering child psychiatrist who overturned the whole approach to behavioural issues by developing a "Client Centered" approach - ie starting where the client is rather than where you would like or imagine them to be.
The strength of the model used is that it is totally holistic - it covers every conceivable situation that a consultant or leader will ever encounter,and....it does not have a "sell-by date" - this is not some gimmicky shallow "psychobabble" but rather a deep and considered approach that seeks to help us to understand and work with the very essence of human nature.