In my opinion the best book for aromatherapy is the Complete guide to Essential Oils by Valarie Worwood. You can find used copies on Amazon for as low as $7.49
Link edited by PG Angie
I like Aromatherapy an A to Z by Particia Davis. This was one that was recommended by the aromatherapy instructor in school.
If you have a Half Price Bookstore in your area I would check there in addition to Amazon.