I have a great book - The 'New (1989) all-colour edition' of Food for Free by Richard Mabey. ISBN 0-00-219865-7
Covering a fantastic variety of both edible & poisonous wild - trees, shrubs, herbs, & flowers, nuts, fruits, seaweed, fungi, shellfish. It includes lots of recipes (check-out the Sloe gin), folklore, advice & tips. It's also good to know what to avoid just incase.
As it's one of my most treasured possessions. Some might say that's a bit sad but I don't care ;~}