I think I’ve seen Cunningham’s Encyclopedia of Magical Herbs by Scott Cunningham on the shelf of every Pagan I’ve ever met. Some Christians too, now that I think about it. Whenever I’m researching an herb, the first book I pick up is this one, followed by Cunningham’s Encyclopedia of Wicca in the Kitchen. The next step is most likely to take a break from the books and turn to the internet. More often than not, the links I find mostly contain references to this book. Magazine articles and articles in pagan almanacs reflect Cunningham’s information. You can tell in books which authors have this book in their library before you even read the reference list, his influence is that strong. In my experience, it is the must have herbal for the modern pagan, especially if your interests lie in the direction of magic, alchemy, or anything along those lines in folklore and mythology. If you are interested in the occult lore of plants, you probably already have this book.
On the other hand, if folklore is nothing more than interesting trivia to you, and your interests lay in the practical usefulness, medicinal, or gardening aspect of herbs, this book won’t likely help you much. There is some information about whether or not an herb is safe to use topically or internally, but chances are you can find better sources of that information.