Introduction to Essential Oils by Aromahead Institute
How Essential Oils Work in the Body by Robert Tisserand
USA & Worldwide: National Association for Holistic Aromatherapy (NAHA)
Canada: Canadian Federation of Aromatherapists (CFA)
Australia: International Aromatherapy and Aromatic Medicine Association (IAAMA)
UK & Worldwide: International Federation of Aromatherapists (IFA)
Each of these websites provide lists of certified aromatherapists.