As for the essentials, go to the library and check out a book on essential oils, natural healing with herbals or any number of sources. There are many many good websites out there that go into great detail about the good and bad when it comes to essentials.
Otherwise, my source comes from 7 years of research and soapmaking. Not to mention the thousands of years in research that midwives/herbalists and other such people have done. I've also taken various courses over the years to educate myself because I DO use "active" essential oils, not necessarily for thereputic use, but because I do not wish to do anyone harm.
Our modern medicine has its roots in herbals, and somehow people have this disconnect that essential oils are inert, yet will use caution with a bottle of asprin, Willow bark = asprin.