Herbal Knowledge is a huge realm this is by no means a comprehensive list, but some recommendations that I am familiar with and have found useful and expect these to be widely useful to others, they are not listed in any particular order. There is no substitute for a well rounded library in your house

Henriettes herbal: This is a great all around site almost all in one site  http://www.henriettesherbal.com/  

The Book of Herbal Wisdom by Matthew Wood

Complete guide to Medicinal herbs by Penelope Ody

Encyclopedia of Herbal Medicine by Andrew Chevallier