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.