I recently picked up a copy of the book "Guide to Wild Foods and Useful Plants" from Amazon Guide to Wild Foods and Useful Plants. It was a blind purchase, not knowing anything about the contents other than the author, Christopher Nyerges. I have some other of his books and I trust his advice. For anyone who doesn't know, he's also the editor of Wilderness Way magazine. Here's an Amazon link:Wilderness Way, Volume 14, Issue 3 Issue
The book is mostly geared towards the southwest and some plants are found mostly in California, but, a lot of the plants are found throughout the US so its still useful outside this area. He covers 71 plants in detail about their food, medicinal and other uses. Its very thorough. There are recipes and some very interesting stories about the uses of some of the plants. At the end of the book is an appendix called "safe families" which covers relatively easily recognized plant families that are nontoxic and primarily edible.
This is not an identification guide but a use guide and in my opinion its one of the better ones. I highly recommend finding a copy for your library.