http://www.amazon.com/SAS-Survival-Hand ... dp/0002171
The SAS Survival Handbook is awesome, I read it from cover to cover many years ago. It goes into what you can eat somewhat but also covers how to test plants for edibility if you get that desparate.
You'd have to be pretty badly lost to be in dire straits though. In spit of the highly publicised daily nutrient intake requirements the human body as with many other animals can stand short fasts extremely well and even extended ones with little difficulty.
I also read elsewhere that grass is the one universal food.. it is everywhere and safe for pretty much any animal to eat. (not sure about those who are appendix-less though) Baby fern frond shoots are edible. In some regions now we have pine nuts that are available at certain times of the year. Field mushrooms, the ones that look exactly like the ones we buy in the supermarket are fine. But remember the warning with mushrooms, don't touch anything you are not absolutely sure of! And don't eat the funny coloured ones! We could survive by foraging out there though it would probably take a life or death situation to get us to try!
The SAS book goes in to methods of trapping if you're interested in eating the wildlife. Also netmaking for catching fish etc. It does come with a warning of a small group of trappers who could find nothing else to eat for 6-9 months from memory but rabbits.. and they died of starvation! Apparently the rabbits were lacking one essential nutrient which was steadily stripped from the body until death occurred. Go figure.
And never stray far from fresh water.
I've done two survival courses and always wanted to do one that focussed on wild foods. The courses are a real hoot I'd recommend them to anyone and everyone.