I agree with you totally about the other foodstuffs - you mentioned sorrel, which I will be posting about in another context. Richard Mabey's now venerable but still excellent Food for Free, and also another excellent book that I've mentioned here before, Medicinal Plants in Folk Tradition - An Ethnobotany of Britain and Ireland, by Allen and Hatfield, are both based on ancient knowledge and usage. These two books also compliment each other, as some foodstuffs available to our forebears also have medicinal properties.. what was commonly consumed could be multifuntional. Both have excellent bibliographies for those who wish to study further.