For a comprehensive introduction to the history and usage of herbal medicine, I suggest looking at the Encyclopedia of Herbal Medicine by Andrew Chevallier.  Having been fortunate enough to have Andrew as a lecturer at Middlesex University, I can fully appreciate his depth of knowledge, which is translated into an interesting, clear, and informative book.

 

This was one of the first books I ever bought on the subject of herbal medicine, and it really helped fuel my interest.  I used it to provide a clear overview of medicinal herbs whilst at university, and still use it as the basis of talks today.    Whilst not as in depth as some works, it would be the one that I would recommend as an introduction to the subject.

 

The book was revised in 2000, and can be viewed at: