My favorite book on herbal medicines in "Herbal Medicine" by Dian Dincin Buchman. The book is well written, contains the appropriate cautions in using herbal remedies, and is clear in its instructions on how to make the preparations.
And if you're also interested in magical herbalism I would recommend "The Master Book of Herbalism" by Paul Beyerl
In general, if it's medicinal herbalism that interests you I would avoid the books on herbalism generally found in the magic/Pagan sections of books stores or published by publishers in this genre. They often gloss over the potential health problems that can occur with using herbs as medicine, and in some cases recommend herbal remedies that could prove deadly.