There are a small number of classic texts that are worth adding to your collection. There are also a huge number of books that may contain some new information but may be a retelling of the classics. We would recommend :
The Encyclopaedia of Herbal Medicine This is a classic of it's kind - the herbalist's bible. Easily laid out, comprehensive, interesting and authoritative. Not a straight through read but rather a chance to look into herbs, ailments, the law or medicines. Simply flick through the book and find a page to suit you.
ISBN 1-85487-586-8 £12.99 (Paperback) 1995
Herbal Teas The perfect introduction to herbal teas, short concise and inexpensive.