Everything changed for me as a parent of a challenging child when I read Transforming the Difficult Child by Howard Glasser.  Although I'm not fond of the title, I find it provides strong foundational knowledge of the approach.

 

Published more recently, Transforming the Difficult Child Workbook has all of the same content as the one above with the addition of lots of case study examples, exercises and space to hone your understanding and practice your skills.

This book, All Children Flourishing, was inspired by the parents of difficult children who came to Howard and said:  "Not only is my intense child experiencing a turn-around, but my average child is absolutely flourishing!"  This book is for parents - plain and simple - and yes, all children are intense at one time or another!  I love the way he presents the Nurtured Heart Approach in this book, particularly the question and answer section at the back.