For a recommended book list, look here.
The Herbal Academy has several articles on essential oils, several of which I and another aromatherapist, Erika Galentin, wrote as a foundational series. To access this series, start with Basic Essential Oils for Daily Living. I will be writing a new blog post for them each month, so keep checking back. I will also list them here.
Andrea Butje writes informative articles and provides safe, effective recipes.
Plant Therapy blog:
A wonderful plant directory at The Plant List.
Empowered Essential Oil Living - this is actually my group. The goal is to share safe, natural health options, particularly essential oils. I also have a business page: Golden Heart Essentials.
Robert Tisserand Essential Training
Essential Oil University
Essential Oil Consumer Reports
FREE Introductory Essential Oil classes:
Aromahead Institute offers a free introductory class. Their other programs are also excellent.
The School for Aromatic Studies also offers a free introductory course.
Other classes and webinars:
Robert Tisserand offers several great courses on his website the Tisserand Institute.