If you go down to the "search" function, click "this forum"' and type in the word "books" you will find several Herbalism threads on the subject.

Grieve is my favorite--when I found her compendium I thought I had died and gone to heaven. She was published by Dover in two volumes, and another company combined them into a hardback more recently.

If you are more interested in what has been researched more extensively from a medicinal point of view in modern times, for actual use of herbs, there are other books that might be simpler and safer to rely on. I found a new one a few months ago that lists a very small number of herbs that are probably quite safe to use (If I find it I'll list the name). The older herbals should not be depended on exclusively, as modern research has determined that some traditional herbs can contain dangerous constituents. Additionally, some of the modern books on herbs are written by people that really don't know of the hidden dangers, so you have to be careful to check the credentials of the authors. Not everyone that has written a book on herbs is actually a fully credentialed herbalist, and some people that think they are really are not.

Rosemary Gladstar is an actual herbalist, connected to the earth and has been using herbs for decades. She is wonderful. Just knowledge isn't enough to write a book- has this person used herbs? Why and how? What is their philosophy regarding the earth and mother nature? Do they grow their own herbs? Do they give historical info on the herbs as well, such as folklore? Makes for better reading!
But regardless, their are many books out there. Pick and choose carefully, learn about the author first is my opinion.
There are some good books written by Native Americans, who used herbs as food and medicine. There is one by Tis Mal Crow that I like. I'll look at a few of my good ones, and give you actual titles! Best of luck to you on your journey.

dani, here's a good one: Native Weeds, Native Medicine by Marie Miczak, D.Sc., Ph.D.
It's a great starter book, written by a woman of Iroquois descent, who also has extensive knowledge in the field ok pharmacology. It lists some basic native herbs, their uses, cautions. It also has recipes for food, Native American "kitchen remedies", how to grow your own herb garden, etc. It is very inclusive, but fine for a beginner.
Another might be The Family Herbal by Barbara and Peter Theiss.

Stephen Foster has written a good book on the subject ("Herbal Renaissance"). I'd also check out this book (and an introductory article on the subject from the same source).