I did a quick amazon.com search. Most of books that appear under "edible plants of the World" seem to be only well known edible plants meant for chefs and cooks, and not more obscure wild edibles.
There is probably a very good reason for that as the rare and obscure wild edibles are pretty much pointless in terms of calorific or nutritional use for the payback invested in energy terms for processing and collecting. Most 'Wild' edibles worth seeking out aren't really wild, because the plant would have already been exploited, cultivated and used in general agriculture.
Probably the best place to start is gathering knowledge on what you can find, use and exploit out in the wilderness, is down at the local supermarket fruit and veg stall and work backwards in time (a few thousand years) and geography to see if you can find the smaller and less nutritional (energy not vitamins and minerals) version in those 'wild' places.
The other main difference of course, which many Homo Urbanus finds a little difficult to understand is that the seasons play a huge role in the 'wild' foods as the supermarkets don't have seasonal produce anymore as they operate within the global economy.