For those looking for a good foraging book, I highly reccomend 'The Forager's Harvest', by Samuel Thayer. Unlike most edible plant books, this book only covers a few plants(about 30). However it discusses each one in great detail, including how to identify, harvest, process, and prepare the plants for eating.

I have a Petersons' guide and though it covers many more plants, it provides very little useful information. I do not find it very helpful in identifying plants, and it is even less helpful about how to prepare them(usually giving a sentence or less on how to use the plants). Every plant is different, many of them need to be harvested and prepared in a very particular way, and simply saying "use as potherb" is not sufficient.

Also, this is a little off-topic, but for anyone interested in foraging for edible mushrooms, buy 'All the Rain Promises and More', by David Arora. I've looked at many mushroom books and this is the most useable(and entertaining) one I've seen, and one of the best field guides I have among any subject. The book is written for the Western US but many if not most of the mushrooms are common throughout the country, so it is useful there as well.