The Grigson factor lives on. Not just in three decades of brilliant food writing by the late Jane Grigson, but now by her daughter, Sophie Grigson, who is joined in the family tradition by her husband, William Black. Together they have written Fish.

It's a passionate plea to make more sensible use of a fragile world resource. While Sophie takes on the challenge of cooking fish, both strange and familiar, William fills in the background to the varieties of fish and ''how to buy'', as well as providing an insight into the vagaries of the fishing industry.

Working at the buying and selling end for more than 18 years (though he graduated in anthropology), his contribution to the Grigson factor is invaluable. Their intention was to complement, rather than replace, Jane's classic work, The Fish Book (1973).

''When Jane was writing,'' says William, ''Patagonian toothfish and orange roughy were virtually unknown. Even snappers were a rarity then, but these days they are easier to buy than turbot.''

William married Sophie several years after Jane's tragic death from cancer in March 1990, the day before her 62nd birthday. Her entry into food writing had come about by accident, when a friend who was writing a book about French charcuterie gave up and Jane took over. She had taken an English degree at Cambridge and had only an elementary knowledge of cookery. Her career had, until then, been in art galleries and publishers' offices as a picture researcher and translator.

After the charcuterie book in 1968, the Observer asked her to write a food column and she used to speak about those days in her frank, amusing, down-to-earth way, when her husband, the poet, critic and naturalist Geoffrey Grigson, was such a vital support.

After he died, in 1985, Jane noted: ''Geoffrey always said that people wrote best about what they knew best. But it was when they tackled what they did not know that they began to sound pretentious and ridiculous. The word fake hung over me. Luckily, he came to my rescue. My first subject for the Observer column was 'the strawberry'. Right, he said, we'll find out what the strawberry has meant to people, what they have done to it, how they have developed it and so on. Somehow we got through. I cooked the recipes over and over until he was satisfied.''

While Elizabeth David had, by this time, challenged British palates with good tastes from the Mediterranean which lacked counterparts in post-war Britain, now Jane began her long love affair with ingredients.

More that anyone else, she made us look at the basis of good food through the raw materials. She led the movement against intensive farming and chemical pollution, which is no longer a cranky minority concern. We have to fight, demand, complain, reject and generally make ourselves unpopular, she used to say, otherwise our children and grandchildren will never know the delights of fresh, seasonal food.

I first met Jane in 1981 on a trip to Aberdeenshire to investigate the qualities of Aberdeen Angus beef. All day we trailed round farms, slaughterhouses and butchers' shops; discussing, questioning, listening. From her books she had sounded so scholarly and knowledgeable, yet here she was joking with the North-east farming worthies whom we met; her Northern English origins in sympathy with theirs, her deep, bubbly, infectious laugh making her so approachable.

The Grigson factor, as it was then, applies equally well today: shopping daily for the best things you can find, the most honestly produced, the finest tasting ingredients, and doing as little as possible to them.

''Our dishes are not, on the whole, complex,'' she used to say. Perhaps they bore us a little. But try making them with the best of everything and they come alive again.''

The problem of finding the best of everything today remains.

In the case of fish, many traditional fishmongers have disappeared and the quality of fish in supermarkets is often far from the best. But in the last 20 years the fish sellers, according to William, are slowly beginning to improve. He believes the momentum is there to improve even more. But more and more people must carry on complaining when they see horribly stale fish on display.

''Cultivate your suppliers,'' he says. ''Work with them. Find out what is around and possible, rather than out of season and impossible.''

The complexities of the modern fishing industry are not easily understood, but one of the insights of Fish is it's clear explanation of the issues: sustainability; good and bad fishing methods; the nature of fish; and the future of aquaculture.

To understand fully, it's also necessary to go back to the origins of fish-eating. Follow the development from shellfish-eating nomads through the fasting Christians, who believed fish was good for the soul since they were cold-blooded and ''cooling'' to eat.

Sandwiched between William's treatise on fish are Sophie's recipes. While he admits to being a tad puritanical in his tastes, she is the opposite. Part of the Grigson factor also involved adventuring with your palate. Her first memory of the beauty of fish, she says, was when her mother took her as a child to the fish stall at the Wednesday market in Montoire, near their holiday home in France, and taught her to love fish.

l Fish by Sophie Grigson and William Black is published by Headline at #20

l Sophie Grigson will demonstrate fish dishes from her book at Valvona & Crolla, Edinburgh, at 7pm on May 12, and at Waterstone's, Glasgow, at 12.30pm on May 13