Foraging is increasing in popularity and there are more and more courses all over the place, have a search and you should find one near you. Sorry if my earlier post was a bit drastic but there's plenty of self proclaimed experts online who could easily get folk ill.
The best field guide I have found yet is: Collins Nature Guide - Wild Flowers of Britain and Northern Europe
http://www.amazon.co.uk/Collins-Nature-Guide-Flowers-Northern/dp/0002199963
It has very detailed discriptions (habitat as well as plant) along with colour photos which is suprisingly scarce in foraging guides considering how important correct ID is. It is very easy to use as it is colour-coded (by flower) and not alphabetically listed by latin name like most are which is a real pain. The name is slightly misleading as it packs in loads of trees and shrubs as well as info on culinary, medicinal and other uses (dyes, strewing etc); more importantly though- whether the plant is poisonous or protected.
There are better books out there for foraging receipies (and seashore foraging) but this is the best I have come across yet for a field guide as the ID is detailed, you get loads more info to boot and it fits in a pocket. You can still read it as a book due to all the extra info on top of ID, unlike..
Larousse Pocket Guide to Wild Flowers of Britain and Northern Europe
http://www.amazon.co.uk/Wild-Flower...=sr_1_1?s=books&ie=UTF8&qid=1327959102&sr=1-1
Which has even more detailed discriptions, it does uses sketches but these are good and give loads of detail with the text. It offers no other information though and cannot really be read as a book in its own right. It also has the handy colour coding too.
I'm lucky enough to have found a retired botanist turned proffesional forager who is happy to take me on his wanders but I did help him build his house and give him a load of wild seeds I'd saved so he could cultivate alot of what he needed on his doorstep.
Happy hunting