I am a novice, but have gotten both "Nature's Garden:..." and "The Forager's Harvest:..." by Samuel Thayer recently. Both seem to be very well-written and very comprehensive. I am anxious to start identifying some of these plants come Spring next year.
Disclosure: Do not eat ANYTHING that you do not 100% identify correctly. Find a mentor to help you on your path. You can probably find one through your county extension service.
The Rodale Illustrated Encyclopedia of Herbs wouldn't hurt to get either-it has many of the wild "herbs" that are edible. Plus knowing some of the medicinal properties of wild plants could come in handy.
Many of the little tiny weeds in your yard that come up right away in the spring have more vitamins and minerals than oranges or even some vitamin supplements.
It also has many other plants listed that you do not want to eat--knowing the poisonous ones as well as the edible ones is just as helpful to learn.