A few useful books available on amazon

-Aromatherapy Workbook: A Complete Guide to Understanding and Using Essential OilsThe Complete Book of Essential Oils and Aromatherapy: Over 600 Natural, Non-Toxic and Fragrant Recipes to Create Health - Beauty - a Safe Home EnvironmentAromatherapy: An A-Z: The Most Comprehensive Guide to Aromatherapy Ever Published

I found this blog particularly useful for my self and would recommend other readers to follow it

http://shirleyprice.blogspot.com/2010/04/holistic-health-2010-occupational.html

Reference books which could be consulted

CLARKE, S. (2009) Essential Chemistry for Aromatherapy 2nd Edition London: Churchill Livingstone

LAWLESS, J. (2002) Complete Illustrated Guide to Aromatherapy: A Practical Approach to the Use of Essential Oils for Health and Well-being
Element Books

LAWLESS, J. (2002) Encyclopedia of Essential Oils: The Complete Guide to the Use of Aromatic Oils in Aromatherapy, Herbalism, Health and Well Being Thorsons

PRICE, S. (1998) Aromatherapy Workbook: Understanding Essential Oils from Plant to Bottle Thorsons