When choosing an essential oils book, consider both your level of experience and what you want to learn. Some books offer more or less background information, while others are mostly a collection of aromatherapy recipes.
Here are the aromatherapy and essential oils books I have found to provide the best overall information. For more in-depth information, consider an aromatherapy education program.
For beginners, Aromatherapy for Dummies by aromatherapy expert Kathi Keville offers good, basic information, along with some recipes, but isn't the best written book. A better book is the second edition of Aromatherapy: A Complete Guide to the Healing Art by Keville and Mindy Green.
Authentic Aromatherapy: Essential Oils and Blends for Health, Beauty, and Home by Sharon Falsetto, a UK-certified aromatherapist, is a good choice for essential oil profiles but doesn't have many recipes.
For profiles, along with recipes, check out The Heart of Aromatherapy by Andrea Butje, a clinical aromatherapist and founder of the Aromahead Institute.
Essential Oils Every Day: Rituals and Remedies for Healing, Happiness, and Beauty by Hope Gillerman is a good alternative if you want a detailed program to follow. Gillerman lays out essential oil programs with various objectives: better breathing, relaxation and focus, better sleep, skin care, travel, and inspiration. She starts with five basic oils and then offers alternatives to help you build your essential oil collection. Gillerman is a long-time practitioner of holistic healing techniques, including aromatherapy, and founder of H. Gillerman Organics.
Valerie Ann Worwood is a well-respected aromatherapist who has written a number of aromatherapy books. Possibly her most popular is The Complete Book of Essential Oils and Aromatherapy, which provides more than 600 natural, non-toxic, and fragrant recipes.
I particularly like Worwood's more specialized books: The Fragrant Heavens, later republished as Aromatherapy for the Soul, which explores the spiritual and energetic dimensions of aromatherapy. The Fragrant Mind looks at aromatherapy for personality, mind, mood, and emotion.
Using essential oils during pregnancy: Aromatherapy and Massage for Mother and Baby by Allison England, an aromatherapist with personal experience working with pregnant women and new mothers.
Using essential oils with animals: Holistic Aromatherapy for Animals by Kristen Leigh Bell.
Using essential oils from the perspective of Traditional Chinese Medicine: Aromatherapy for Healing the Spirit by Gabriel Mojay.
Using essential oils with acupressure, Acupressure with Essential Oils: A Self-Care Guide to Enhance Your Health and Lift Your Spirit by acupressure practitioner Merina Ty-Kisera.
Subtle/spiritual/vibrational use of essential oils: Subtle Aromatherapy by Patricia Davis is the classic. Also see Aromatherapy and Subtle Healing Techniques by Joni Keim and Ruah Bull and The Essential Guide of Aromatherapy and Vibrational Healing by Margaret Ann Lembo.
Ayurvedic use of essential oils: Ayurveda & Aromatherapy by Light and Bryan Miller.
For information on the chemical composition of essential oils, take a look at Kurt Schnaubelt's Advanced Aromatherapy: The Science of Essential Oil Therapy. Also, Schnaubelt's newer book, The Healing Intelligence of Essential Oils, is a more complete resource with a broader range of technical information and more aromatherapy blends.
For comprehensive profiles of essential oils, pick up Aromatica: A Clinical Guide to Essential Oil Therapeutics, Volume 1 and Volume 2, by Peter Holmes, or The Complete Guide to Aromatherapy, Third Edition, by Salvatore Battaglia.
For the healthcare professional or really serious reading, there's Aromatherapy for Health Professionals by Len and Shirley Price.
Another important essential oils book is the second edition of Essential Oil Safety by Robert Tisserand.
When browsing for an essential oils book, look at the qualifications of the author. Anyone can publish ebooks on Amazon Kindle, and I've seen plenty of ebooks with questionable or incomplete information.