It you're interested in any aspect of the application of NLP in a training or teaching context don't even think about looking at anything else until you've read this book.
Although the style is, as in Introducing NLP, a little dry, this book reaches the parts that other books don't even mention. Both Joseph O'Connor and John Seymour have years of experience in this field so they tell you what you really need to know.
After 5 years reviewing books on NLP I have yet to find one which beats this... more info