There is much good literature about essential oils. However, the best book about essential oils is one that is realistic.
Too often, many books will pitch essential oils as cure all elements. This is not realistic. Essential oils can be dangerous if applied directly to the skin. Some cause extreme photo sensitivity and can cause second degree burns to the areas they are applied to once someone is exposed to sunlight.
Consider the best book about essential oils one that is truthful with reasonable claims. Essential oils have been shown to be excellent to enhance a mood, but they are little more than aromatherapy. Some oils such as lavender can make people feel relaxed and even sleepy, for example.
The best book about essential oils will not be affiliated with any marketing scheme to sell the oils. Just make sure to be discerning when picking out literature regarding these popular products.