There are several reference books on Essential oils. My only complaint about EO's is that they are typically marketed as healing powerhouses. Particularly by those who are trying to sell you their MLM version. If you were to look at health as a big puzzle with many pieces, there would be large pieces and small pieces - the large ones representing what is more important. Large pieces would include good nutrition, exercise, getting enough sleep, and having good healthy relationships. Smaller pieces to the health puzzle would be things like music, leisure activities, and I would put as a small piece essential oils. Do they have an effect on the total health picture? Yes they can, but which is more important - using essential oils or changing your diet to whole natural foods - staying away from processed, chemical laden products.
I remember talking with a patient who was emphatic that all of her food was organic and of the highest quality - which is great - the inconsistency was that she smoked a pack of cigarettes every day.
So EO's are good, but if you are not willing go get off the coke and twinkies, expecting the EO's to bring you to the next level of health - who are you trying to kid. Right?