Here are books and supplies that I have found invaluable for my aromatherapy practice:
The Fragrant Pharmacy by Valerie Ann Wormwood
This was one of the first books I bought when I started working with essential oils. There are a large number of recipes and uses of essential oils in the book and Valerie has a clear writing style. She does tend to use a large number of different oils in her recipes but it is still worth a place in your essential oil library.
Encyclopedia of Essential Oils by Julia Lawless
Julia Lawless' book is my essential oil bible. It is an A-Z of over 160 essential oils and describes, in detail, the oils Latin name and family, a description of each oil, how it is extracted and what it can be used for. If you are serious about learning more about essential oils then this book is a must.