Fit for Life, by Harvey and Marilyn Diamond is an Excellent Start. A very good gentle introduction to Natural Hygiene. Fit for Life, Part 2 is a good follow up
Dr. Keiki Sidhwa's book "The Quintessence of Natural Living for Health and Happiness" is an excellent read. Available from me, email for details.
Read any book by Dr. Herbert Shelton you can get your hands on. They are the classics. In particular - The Science and Fine Art of Natural Hygiene, The Science and Fine Art of Fasting, The Science and Fine Art of food and Nutrition, and Fasting can save your life.
After all this, if you still want to get more knowledge, in a systematic manner, that will help you provide nutritional counselling (and no more, mind you), you can do the Natural Hygiene Correspondence course at www.transformationinst.com. THis is orignally TC Fry's course, but it has now been revised by Dr. Robert Sniadach. I am doing the course myself and it is excellent and highly recommended. I have only two reservations about this course - Fry's insistence on fruitarianism and food combining. If you take these with a pinch of salt