THIS is a book about the things we don't know: about a species of knowledge which, until very recently, most people took for granted. It is a book about land, food and farming: three things which continue to keep us alive, but of which we are now mostly ignorant.

The loss of this knowledge, as Barbara Kingsolver writes of the US, has had a major impact.

"Most people of my grandparents' generation had an intuitive sense of agricultural basics: when various fruits and vegetables come into season, which ones keep through the winter, how to preserve the others, " but now, "This knowledge has vanished from our culture."

In a way, Kingsolver is a representative of this transition: a novelist and intellectual who has lived in many parts of the world, she is also the child of a farming family.

For 20 years, her husband has owned a farm in the Appalachian Mountains, which they have migrated to for holidays. Now, her family decides to take the plunge. They move from Arizona to the farm, and set themselves a task: for a year they will survive on food they have grown or raised themselves, or bought from the local area.

Animal, Vegetable, Miracle is a strange hybrid of a book. Most of it is Kingsolver's account of her local food year, but stirred into the mix are sidebars by her husband about the politics of food and farming, and recipe boxes by her daughter, Camille.

Add to this chapters on her trips to Italy or Massachusetts, and what she ate there, and you end up with a structure whose parts are often fascinating, but which somehow doesn't hang together.

Nevertheless, there is much worth reading, above all her eloquent and incisive dissections of the lack of interest shown by most urban Americans in their cultural and food heritage. There is an elegiac quality to her writing about the disappearance of America's farmers, and the crazed politics and economics sealing their fate. She quotes the American farmer-novelist Wendell Berry, who writes that eaters must understand that "how we eat determines, to a considerable extent, how the world is used".

If eaters did understand, what would our careless, bargainobsessed food-buying habits say to our remaining farmers? "Let them eat dirt, " suggests Kingsolver. She and her family are no longer content to eat dirt.

Reading her account of why, it's hard to see how anybody could be.