This book clearly outlines many essential oils, their sources and recommended uses. There is also an invaluable section listing common ailments and which essential oils are best used to treat them. This was the first aromatherapy book I ever purchased, and I still refer to it often.
I have a number of aromatherapy books, but this one is my favorite. It's a small book but it contains all the practical knowledge you need to know to practice good aromatherapy. I have found it invaluable and highly recommend it.