What makes a cookbook a classic? Immediately we think of the recipes; all that delicious food, easy to cook, impressive to serve. A recipe, though, is only a list of ingredients and the method of combining them. Recipes are like plot summaries of Shakespeare’s plays; we know what’s supposed to happen but the real pleasure is in the writing.

And it is the same with a classic cookbook. What makes our mouths water when we read a cookbook isn’t the food on the table; it’s the story about the food on the table. Pleasure always starts in the mind – it makes no difference whether the pleasure is a day out, a wonderful meal, a visit from a friend, or sex. In fact food and sex go together so well because both are anticipatory. In the imagination nothing goes wrong.

Open Nigella, and even if you have never done more than cheese on toast, already, you’re listening to her voice. Nigella cookbooks are intimate – something like a diary. Confessionals of failures, flaws, greed, indulgence, accidents good and bad, the joy of doing it right. Of showing off for friends. Of sitting alone with a plate piled with lemon linguine.

And the cookbooks are voyages of discovery; of raw materials that become philosopher’s stones. Of fire, ancient and suggestive. Of breaking bread and gathering together, and of the pleasure of shopping for food like an explorer – fresh, loose, earthy, sensuous, not shrink-wrapped and pre-packed.

Food, like language, is a basic everyday necessity. We need to communicate. We need to eat. Writing about food uses one simple basic – language – to talk about another simple basic – food. And that’s where the magic starts. Just as in the fairytales, with their self-sweeping brooms and geese that lay golden eggs, the ordinary everyday stuff of life turns out to be the treasure under our noses.

Many perfectly good cookbooks offer up their recipes simply, briskly and clearly. No commentary and nothing personal. And that’s fine. We’ve all dashed to Google to check a method we half-remember or to compare recipes for bolognese. The classic cookbook is different. The classic cookbook is practical, for sure, but it’s more than a “how to”. Nigella calls her recipes, “a reminder of possibilities”. And that’s what a classic does – it doesn’t matter when it was written, it doesn’t matter how styles have changed. It doesn’t matter if the writer is dead or alive. Why not? Because the language is alive.

It doesn’t even matter whether or not you actually cook any of the food. If I cooked anything from The Alice B Toklas Cookbook I would soon be dead of sclerosis – every artery in my body crammed with cream. But I love to read her. When I read Nigella, I’m there for the story. What does she have to tell me about this dish?

She writes: “I first had salsa verde when I was a chambermaid in Florence.” Then I go on to discover that she ate it in a trattoria where most of the diners were transvestites – and I’m at the long table with her, and the burnished blond bombshells she describes, and we’re dipping our bread into – listen to this – the “deep flavoured spiky sauce the colour of snooker-baize”.

Nigella is funny. The writing rolls along and then there are the one-liners, a store cupboard of them. A pinch of Woody Allen: “Christmas is like the country; not much to do apart from eat and drink.” A dash of Oscar Wilde: “Being right isn’t everything.” A drop of Joyce Grenfell looking straight to camera and saying: “Remember that defrosted strawberries take on the texture of soft, cold slugs.” When she’s writing about feeding children I can’t decide whether she’s Mary Poppins, kind but firm – “It’s never too soon to get a child used to pink lamb and blue beef,” or whether she’s channelling Roald Dahl: “A pan so big that both the children could fit in it together – and have the lid put on too …”

When How to Eat was published 20 years ago, the title told us what to expect – that this was not only about the pleasure of cooking, but a reminder that cooking is not an end in itself; we cook because we like eating. The title put the book into the timeless category. Everyone needs to eat – but do we really know how? Eating, like love, like language, comes naturally, but also needs to be learned – and especially if we want to learn to do it well.

How to Eat promised, and delivered, far more than recipes; here was a practical philosophy – a way of getting more enjoyment for yourself and others. Food, for Nigella, becomes a way of talking about what is good and what is not, in the deepest sense of what nourishes us and what doesn’t. Why else would she call a recipe “Spring Lunch to Lift the Spirits?”

That simple lemon linguine, by the way, is just gorgeous.

• A 20th anniversary edition of How to Eat is published by Vintage Classics.