If you're into the voice, especially singing (in any style), you have to get this book. Based on Jo Estill's methods (and a small section on Accent Method/Janice Chapman breathing), it really clearly defines how the voice works, how you can change it, what each change does and how you can use it. It's just a must for all singers and teachers.
Almost everything (excluding the breathing) is based on tons of high quality research, and helps to encourage students to gain control over their instruments and be able to change their voice quickly. No endless scales, just informed choices! A CD is also available if you're having difficulty creating the sounds, or if you haven't been on an Estill or a Vocal Process course.