In this book Sellar has profiled 83 essential oils in quite a lot of depth, devoting two pages to each oil. After the usual list of names in English and Latin, extraction method and so forth, the categories listed are: aroma, features of the plant, history and myth, chemical constituents, properties, precautions, effects on mind, body, and skin, and good blending partners. This is pretty much the entire book, although there is a brief glossary at the end which runs along the following lines:
Antidontalgic: Relieving toothache.
Cajuput, Cinnamon, Clove, Nutmeg, Peppermint, Pimento.
There are also a couple of charts showing blending families (citrus, herbs, spices and so on) and suggested oils for skin types.
For someone who already has a basic working knowledge of aromatherapy, this is a very useful book, combining each of reference with impressive depth. I would not recommend it as the first book for a beginner, however. Sellar assumes that her readers already know how to use essential oils so does not provide any information on blending, carrier oils, methods of application, massage and so forth. In addition, the referencing tends to be one-way: it is much easier to look up an oil in this book than it is to look up a symptom. So if you want to find a good oil for, say, PMS, this book will be much harder to use if you do not already know that you should be looking at lavender, geranium, clary sage etc. The glossary at the end is of limited use and certainly does not constitute a materia medica. I find this book most useful when used in conjunction with other aromatherapy books, so that I can cross-reference, usually by looking up symptoms in other books and then turning to this one to find out more about the oils that have been suggested.