This is the book that started me on my journey "The Art of Aromatherapy" by Robert Tisserand, His website:
http://roberttisserand.com/, and is still my favorite reference as it covers a wide range of very practical uses for essential oils, he explains the unique as well as the common properties and includes some history, it is very thorough.
My first oils were from Frontier Herbs, very good quality. They have merged their essential oils with "Aura Cacia" which had very good quality too, and are now a part of the Frontier Herb's company. I have used them for over 20 years and still have 5 of my original 6, though they are nearly empty. True essential oils do not go bad, though the quality may change either improving as with Patchouli and Ylang Ylang, ...I know there are some that weaken but I don't know which ones.
http://www.frontiercoop.com/, and
http://www.auracacia.comFor very high quality I use Swiss aromatics.
My two favorite books for the medical use of essential oils is by Dr. Kurt Schnaubelt, they are Advanced Aromatherapy, and Medical Aromatherapy both very good, but a strong understanding of chemestry, better yet Biochemistry is a big plus. His website's:
"Pacific Institute of Aromatherapy" and "Original Swiss Aromatics", founded by Dr. Kurt Schnaubelt, 1983,
http://newpacificinstituteofaromatherapy.com/about-us/ In a pinch "Now" brand essential oils available at your local health food store are good to use, their quality is better on some oils than others, and more reasonably priced. They are slowly getting into organic oils. I have had very good results with their Oregano, Tea Tree, Pine, and all the spice oils.
I hope this helps someone, or if Sunshine is still looking for more information.
Evelyn