I carry laminated pocket field guides for Edible Plants, Mushrooms, Medicinal Plants, and Animal Tracks. There are quite a few others as well but I haven't gotten those yet. Several of which are concerned with bird watching and other topics.

You can find those in the "Gardening" section @ Barnes and Noble, Books A Million, and many other book stores.

Per IzzyJG, you should definitely check out Blast's Edible Plants Blog.