In the forests of birch and pine, shafts of autumn light fall through the branches and lie in puddles on the mossy, mulchy floor, and you can see them, smell them, almost feel them growing and rotting and dying all around. The wood is full of mushrooms and fungi. There are brick-coloured, bay or sinister purple boletus with slimy caps; cream and beige puffballs sending up a mist of spores when you touch them; lilac blewits, the red-topped beechwood sickener, copious rings of fairy mushrooms and trooping crumble caps, scarlet warty fly agaric. Black, eerie green, dull chalky white, pale brown, greasy yellow, grey, buff; mushrooms shaped like cauliflowers, like UFOs, like dead leaves, like spiders' webs; high in trees or under logs, hidden in the earth or erupting in ditches. They often have human names (the prince, the blusher) or names of body parts (the shank, the foot), and can look uncannily like flesh and entrails - pus-filled boils, scabs forming along logs, dead fingers flopping in the undergrowth, vital organs spilled out onto the grass, giant yellow ears listening on tree trunks, tumours among the roots, pieces of disintegrating flesh sliding down the hummocks, piles of fetid dung, snotty gunk in the earth's crevices - and like dead flesh they rot, flyblown and maggoty, sending up rich strange odours: almond, coconut, geraniums, apricot, sickly sweet meat - a stew of compost, a fruity sweet decay.

It is no wonder that many people find these slimy, rotting growths repellent, while others (like me) adore them and can become obsessed. Unlike most plants, mushrooms do not have chlorophyll. Rather, they are like animals, and take their nutrients ready-made from plants and even from animals. They are parasites and live on dead things. To reproduce, they drop and shoot spores. They grow in dark, hidden places - and they grow quickly. You can spot a scattering of tiny nubs on the soil one day and the next day return to pick them. Standing in a Swedish forest in autumn, it doesn't feel at all surprising that these fungoid growths can kill.

The most common poisonous mushroom is the fly agaric, a powerful hallucinogen used by the Lapps, and also by reindeer: you're not likely to eat it, though, because it looks so dangerous, its redness sending up warning signals (although its smell is faint and it tastes quite pleasant). Its name comes from the practice in medieval times of breaking it into pieces and adding it to milk to stupefy flies. Eat it, and the symptoms begin 20 minutes to two hours later: the central nervous system is affected, the muscles pull and twitch spasmodically; then comes dizziness, deep death-like sleep, although it doesn't usually kill.

The writer Nicholas Evans, along with his wife and his brother- and sister-in-law, very nearly died last month when they ate Cortinarius speciosissimus, otherwise known as fool's webcap (in my mushroom bible, Mushrooms and other Fungi of Great Britain and Europe by Roger Phillips, the edibility of many cortinarius is 'unknown' but the speciosissimus is recognised as dangerous). It has a reddish-brown cap and rust-coloured gills, and can apparently be mistaken for a chanterelle. It is found mostly in coniferous woodlands in the Highlands - which is where Nicholas Evans picked it. Eating it is often fatal: it attacks the kidneys (Evans suffered acute kidney failure) and spinal cord, and breaks the liver into pulp. He and his mushroom-picking companions were lucky to survive.

A couple of weeks later, a woman on the Isle of Wight wasn't so lucky - she died after she and her companion ate the death cap (Amanita phalloides). This is Britain's most deadly fungus, responsible for 90 per cent of death by mushroom poisoning. It grows in deciduous woodlands and really doesn't look too bad: it has a greenish-yellow cap, convex at first and then flattened as it grows; a smooth white stem, white flesh and gills. I have no idea what it tastes like, but if you eat it, it takes between six and 24 hours before there are the first symptoms of violent diarrhoea, vomiting and stomach ache - and then (this is the cruel part) apparent recovery. But the poison is secretly at work, attacking the liver and kidney and being then reabsorbed into the blood to circulate once more, causing repeated damage.

The trouble is, the death cap looks to me exactly like hundreds and thousands of other fungi with white gills and brownish caps. I'm no expert on mushrooms, just an avid amateur. I strictly avoid anything with white gills. I even avoid touching them. I don't pick anything that I'm not 100 per cent sure of, and therefore, when I go mushroom hunting, I look for only half a dozen or so of the many thousand fungi that can be found in Europe.

On this occasion, in the deep wood in Sweden, I am looking for ceps, chanterelles, horn of plenty and a small brown mushroom with a funnelled cap whose English name I don't know, but which grows plentifully through the cool damp autumns, like drifts of sodden leaves. Ceps (also known as penny buns) are the magnificent kings of mushrooms, firm and stout with a brown cap and white flesh. They look robust and taste almost meaty, with a mushroomy, Marmitey smell when dried (we have bought a drier from Switzerland, with stack of shelves on which we place thin slices of cep before turning on the humidifier). One sniff and I'm back in the mysterious forests, light falling on to soft mossy earth. Some years, they are hard to find here; others, they are plentiful. One of the points of mushroom-hunting is that you never know where you'll find them and there are days when you return empty handed. And if ceps are the kings, chanterelles (which Nicholas Evans apparently thought he was picking) are the queens: they are a lovely egg yellow, with a tapering shape and gills running from the stem to the slightly depressed cap. When they are old, they become large and soggy, grime sticking to them making them hard to clean. They are often found in patches along paths, but in spite of their yolky colour, they can hide from view, under moss or roots. Horn of plenty is the hardest to find: they are small, tubular (hence the name - though some know them as trompette d'amour), and lie on the ground in thin, leathery crisps, almost indistinguishable from dead leaves. When we're lucky enough to find them we cook them with garlic and serve them on brochettes.

I grew up picking wild mushrooms, and many of my earliest memories involve looking for mushrooms with my parents, stumbling through woods after them on drizzly October afternoons. Other families went for walks, we went for hunts. My father was always an inveterate experimenter, and there was something slightly reckless about many of the mushrooms we picked and brought home to be laid out on the kitchen table like specimens in a laboratory. He would take out his mushroom book, put on his reading glasses and examine each mushroom in turn, holding them up, breaking the cap off the stem, cutting open their flesh, smelling them and even - or is this a false memory? - taking tiny experimental tastes. He had a blithe confidence in his deductive skills, and with an anxious kind of resolve, we would eat anything he pronounced edible. Puffballs, even if they were losing their essential firmness. Common ink caps, even if they were opening out from their ideal ovoid shape and beginning to blacken and stain at their skirts (Ink caps should never be eaten with alcohol, as they can make you very sick - indeed, Roger Phillips writes that they have been given to alcoholics to cure them). Parasol mushrooms - which are delicate and delicious but which, to me anyway, look too like death caps to be sure of. St George's mushrooms, though again, they are white-gilled and I still need to be with my father to be sure of them.

It wasn't just the finding, but the preparing and cooking. You have to get them home quite quickly - mushrooms grow fast and they rot fast; if you put them in a plastic bag they will soon turn slimy - and then brush off all the soil and cut away the woody or maggoty bits before cooking. Mushroom risotto, mushroom with pasta, mushroom in casseroles, mushroom soup, mushroom sauce, mushroom pie, mushrooms on toast, or simply mushrooms - as an accompaniment or, in the case of a meaty type like cep, as a whole meal.

It's usually something you have taught to you, and then it's something that you pass on, like a secret knowledge. When my husband, Sean, and I met, we discovered that one of the things we had in common was a childhood spent hunting for mushrooms - he in Sweden, where he spent every summer with his extended Swedish family, me in Scotland and in Worcestershire, where I grew up. The first thriller we wrote together as Nicci French begins with a family mushroom hunt, and perhaps this was appropriate - mushrooms grow in shadows and darkness; mushrooms and death go together very well. As our parents had taught us, so we taught our own children about mushrooms, carrying or dragging them through pouring rain into dense forests, showing them where to look for ceps and chanterelles, rooting around in the damp soil for our edible treasure.

Why is mushroom-picking such a glorious pleasure, even an addiction for some? Why is it that during my summer holidays in Sweden, where we go each year, I can spend entire days in the woods, whatever the weather, stooping under trees, pushing through bracken, up to my ankles in mud, bitten by mosquitoes, scratched by brambles, whipped by branches, searching for my supper. It can't just be the hunter-gatherer instinct. In the summer we also pick blueberries, raspberries and wild strawberries, and fish in the lake for perch and pike. Sometimes we manage to eat meals made entirely from what we've caught and foraged, and there's nothing to beat the sense of triumph and virtue and old-fashioned thrift, nor the way that each bite tastes good - this in spite of the fact that pike is a nasty muddy fish that has to be disguised by lots of butter and horseradish before it's fit to eat, and perch gives you mouthfuls of tiny bones; in spite of pieces of bugs and stalks in the pudding. Sean makes nettle soup (good), dandelion salad (bitter), elderflower wine (exploded all over the kitchen floor) and he has even cooked snails from our garden (an arduous and insane process that ended up with dozens of miniature blobs that looked and tasted like the little rubbers from the ends of pencils, floating on a sea of garlic butter).

But picking mushrooms goes beyond the pleasure of food for free. In part this is because you never know if you are going to find them. You know where the raspberry and blueberry bushes are, but not where the ceps will be. They don't grow in the same place twice - although mushroom hunters all have their secret sites, which they don't divulge even to other members of the family; indeed, it's not really good manners to ask anyone exactly where they found their haul, and I always feel slightly aggrieved if I come across others on 'my' particular patch. (A few years ago, one of Sean's Swedish cousins separated from her partner and he, in his final act of revenge, left without ever telling anyone where he managed to find such triumphant amounts of horn of plenty.) There is nothing to beat the satisfaction of coming out of a wood carrying bags full of mushrooms, to see someone else going hopefully in with an empty basket.

And perhaps it also has something to do with the fact that you have to learn about mushrooms. It's not just anyone who can go out and pick them. It's a skill (although not a very hard skill if you only reach the level of someone like me, with a handful of types I can recognise without a shadow of a doubt). It makes us mushroom pickers into a strange little community (if I see someone picking mushrooms then, as long as they're not on my patch, I feel a bond with them).

But I also think that the repellence of mushrooms, their closeness to dead animal flesh and indeed their danger, is part of the pleasure - like eating blowfish. If you go blackberry-picking, you don't worry that the crumble you cook later might cause you to vomit or your kidneys to pack up on you. Mushrooms, on the other hand, are a food that's close to poison; they can be hallucinogenic and associated with curses and magic (hence magic mushrooms); they have a greasy texture that disgusts; they can taste rich and almost rancid. They grow in dark, dank, forgotten places, and even as they ripen they rot and even as they live, they die.

Omelette with cherry tomatoes, shiitake mushrooms and marigold petals

If you feel like having this omelette, don't deprive yourself just because there are no marigolds to hand - I only added them here after an opportunistic forage in the garden! But by the same token, if you find some, perhaps they will encourage you to make this dish. They do look very pretty scattered over the tomatoes and add a subtle peppery flavour.

Serves 2

200g shiitake or oyster mushrooms
olive oil
6 eggs
50ml milk
salt and freshly ground black pepper
6 red cherry tomatoes
6 yellow cherry tomatoes
cheese (optional)
marigold petals from the garden

Fry the mushrooms in olive oil, then drain on kitchen paper and keep warm.

In a bowl, whisk the eggs together with the milk, then season with salt and pepper.

In a separate pan, heat some more olive oil until it's nice and hot, then pour in the egg mixture.When the eggs are nearly cooked through, add the raw tomatoes and cooked mushrooms. At this stage you could add some cheese if you fancy it.

Sprinkle in the flower petals, fold the omelette over and serve.

Luxury three layer parmentier

This type of French shepherd's pie is traditionally a dish for using up leftover beef and vegetables. Here I'm using tinned confit of duck.

Serves 6-8

6 legs of confit of duck
25g butter
olive oil
2 shallots, finely chopped
1 garlic clove, finely chopped
220g wild mushrooms
500g potatoes, peeled, boiled and mashed
500g sweet potatoes, peeled, boiled and mashed

Heat the duck legs, together with their fat, gently in a saucepan. Remove from the fat and pull the meat apart, keeping the pieces fairly chunky and substantial. Reserve. Melt the butter and a little oil in a heavy-based frying pan. Sweat the shallots and garlic but do not colour. Add the mushrooms to the pan and lightly fry until they are golden and slightly crispy. Reserve until required for the topping. In a large gratin dish build up the layers, starting with duck, followed by mashed and sweet potato, and ending with the mushroom mixture. You can serve this immediately if all the ingredients are nice and warm. Alternatively, cover lightly with foil and reheat later in a warm oven.

• Trish's French Kitchen by Trish Deseine (Kyle Cathie, £19.99) is out now. To order a copy for £18.99 with free UK p&p go to observer.co.uk/bookshop or call 0870 836 0885