I can't remember where or when I first clapped eyes on Richard Mabey's Food for Free, which reaches its 40th birthday this year. Did my father own it? Perhaps. It's a book that would have chimed perfectly with his bearded sensibilities (he was a mycologist, with the result that from about the age of six, I knew – and would tell anyone who cared to listen – that the red and white spotted toadstools on which gnomes like to sit are called fly agaric and highly poisonous; yes, I was a weird kid). But anyway, it seems always to have been with me, this book: strangely interesting, slightly bonkers and, above all, weirdly prescient. These chefs who think they're being clever when they put marsh samphire on their menus. Don't they know that Mabey and his followers – Food for Free, which has never been out of print, has to date sold more than 500,000 copies – were pickling glasswort when most people's idea of culinary sophistication was a Vesta Curry accessorised with a little chopped banana?

In his introduction to the anniversary edition of Food for Free, Mabey writes that his book was initially regarded, even by him, as part of the counterculture. "I have a snapshot of myself taken around the time the book was published. I'm sitting cross-legged on the lawn in a kaftan, looking rather smug and cradling an immense puffball." In 1972, foraging for wild food was more an expression of one's politics than a way of reconnecting with the seasons (though some things do not change: it still brings out the show-off in people). This is not to say that the book is po-faced. Mabey isn't in the business of deeming something to be delicious just because 15th-century peasants liked it. Goosegrass – the plant you loved as a child because, on family walks, it stuck to your mum's bum – "makes stringy eating". Tansy, once the most popular of garden herbs, is an "unusually off-putting plant… smells of chest-ointment". Ground elder is "frankly rank". Some of his advice is very solemnly of its time. Should you find yourself without a basket, Mabey suggests, simply "pin together some dock or burdock leaves with thorns". But there are also some fine recipes. One of his favourites is for Pontack sauce, made with elderberries and claret (good with liver). Mine is for beech-leaf noyau, a sweet, green, gin-based liqueur – and one day, after a happy afternoon in some ancient, dappled wood, I will actually make it.

A few years ago, I went to Norfolk to interview Mabey. I remember vividly the lunch he gave me of cold beef followed by a flan made with chestnut flour (dish-cloth damp and an unassuming shade of mushroom, it looked like an earnest version of what your granny might have called "shape" – except that it was delicious). Needless to say, in his felt clogs, Mabey didn't look particularly fashion forward. Nor, I think, is Food for Free the work for which he would like to be remembered (that will surely be Nature Cure, which describes the role of landscape in aiding his recovery from serious depression). Nevertheless, it's incredible the way his first book has moved, over four decades, from what you might call the hairy margins to a point where it can hunker comfortably on the shelf between The Moro Cookbook and Ottolenghi's latest, and out-cool the pair of them. Its influence is everywhere.

Where once you were lucky to find mint tea in a British supermarket, herbal concoctions now fill several shelves – even if, so far as I know, no one has begun commercially manufacturing the delicious-sounding lime flower tea Mabey recommends in Food for Free (gather the flowers in June or early July, and dry for three weeks in a warm room). Pickled samphire has begun appearing as a snack on the bars of some smarter seaside pubs, while at L'Enclume in Cumbria, Simon Rogan makes dock pudding from nettles, bistort and dandelion. In New York for a work trip, I opened a glossy food magazine to find several pages devoted to the city's latest craze: pine needles. Ugh! Toilet Duck, I thought. But Mabey, of course, was first here, too, with his suggestion for oil flavoured with the needles of Scots pine. I've yet to see hawthorn berries on metropolitan menus – they taste, according to Mabey, like avocado pears – or fat hen, hogweed and Good King Henry. But now that even MasterChef contestants are made to put on their cagoules and go a-gathering, it can only be a matter of time.