Eric Carle, the rock star of children's literature, has cobalt-blue eyes, gently accented English, and braces holding up his trousers, which together make him seem like a character from fable; part Raymond Briggs' Father Christmas, part Pinocchio's father, Geppetto. His most famous book, The Very Hungry Caterpillar, has sold 30 million copies, bringing his total to roughly 88 million, a fact that, in his 80th year, still amazes him. Forty years after it was first published, The Very Hungry Caterpillar remains a bestseller, and although it's a quarter of a century since my dad last read it to me, when I tell him I am going to Florida to see Carle, he recites the first lines as automatically as liturgy. "One Sunday morning, the warm sun came up..." Returning to the books of one's childhood will thump you in the heart as much as anything.

Carle lives with his wife, Bobbie, in Key Largo, Florida, in a beautifully designed house they moved to three years ago from Massachusetts. They chose the area for the weather and the property for the romance of a little path that runs from the swimming pool through a tangle of mangroves to the water's edge, from whence the only sign of human life is the tip of a neighbour's jetty. (This being a very nice part of the world, the neighbour is Gene Hackman.) The pool is set in a terrace of white brain coral; frigate birds wheel overhead. The other day, says Carle, he watched a big spider stalk a little one, before stepping in to save it. He looks rueful. "I interfered with nature."

The path from Carle's upbringing in Nazi Germany to the most beloved of children's authors and illustrators is one that, when he cares to indulge in self-analysis, he admits makes a certain sense. The link between the deprivations of his early years and the slant of his artwork, so bursting with light and joy and exuberant colour, is such that, he says, if he had grown up in greater comfort he would probably be "pumping gas" these days. The Very Hungry Caterpillar is only 32 pages and 224 words long, but there is a reason one remembers it so vividly: the bulging salami, the lurid watermelon, the caterpillar itself, with its humpbacked body and impudent expression. He has so few words to play with that every one must count, and so it does. When the caterpillar turns into a butterfly it's a joyful moment, but there's also a lurch; something is lost. How many books for the under-fives have subtext?

Carle is a collagist; he builds up the illustrations with layers of paint and tissue paper before they are photographed, giving the final artwork a depth and sophistication absent from so many dashed-off children's books. Before becoming an author, he worked as a commercial artist in advertising and as a designer at the New York Times, and applied the discipline of those fields to his new work. "I often joke," he says, "that with a novel you start out with a 35-word idea and you build out to 35,000 words. With a children's book you have a 35,000-word idea and you reduce it to 35. That's an exaggeration, but that's what's taking place with picture books."

Caterpillar was Carle's third book after Brown Bear, Brown Bear, What Do You See?, written with Bill Martin in 1967 and 1, 2, 3 To The Zoo, produced a year later. As he became successful, publishers approached him with ideas and commissions but, says Carle, he has always worked for himself. "I cannot do a book that says market research has found that three-year-old girls like the colour red or that boys like tractors. I've been asked so many times to do a tractor book. I could do a tractor book." He shrugs. "But I don't know how to do tractors."

Children write to Carle at a rate of 10,000 letters a year. Some of the letters are funny, some sad, many - Carle is not sentimental about children - merely "pedestrian". He can always tell when a divorce is pending. "The line is, 'Will you be my father?' Something of that sort. And I think, 'Uh-huh, trouble.' " His readers are shrewd critics of Carle's work. After The Very Hungry Caterpillar, he wrote The Very Busy Spider and The Very Quiet Cricket, to which one satirical young fan, observing himself in the changing rooms after swimming, suggested a book entitled The Very Slow Penis, to the author's great amusement. One underestimates children at one's peril.

To deal with the volume of correspondence, Carle has two full-time assistants based at the Eric Carle Museum of Picture Book Art, which he founded in 2002 in Amherst, Massachusetts. Over the years, there has been no great generational gap in the way children respond to him. But between boys and girls? He thinks. Boys, he says, react more strongly to a book called Papa, Please Get The Moon For Me, which he wrote for his daughter in 1986, because "when she was little, she looked out and the moon was low and it was in the dead of winter, and it was in the trees. She asked me to get the moon for her; she thought it was a ball." In the book, the pages fold out to accommodate a long, long ladder that the girl's father extends into the sky to get the moon for her. "And the moon is much too big for him to carry down. But the moon says, 'Every night I get a little smaller.' So when the moon is the right size, he takes it down." Boys, says Carle, react more enthusiastically than girls to the extension of the ladder. He smiles: "Freud never sleeps."

For his first six years, Carle lived in upstate New York with his parents, who had emigrated from Germany. In the mid-30s his mother, homesick, took the family back to Stuttgart at a time when everyone was trying to move the other way. War broke out, his father was drafted into the German army and spent eight years as a prisoner of the Russians. Before the war, he had been his son's great ally, a gentle man, an amateur artist. When he came back, he was broken. Carle turns over the memories of this period slowly. He is very careful to get things right.

His mother was supportive of her son's love of art - "in awe" of it, in fact - but was in other ways a rather distant figure. She was not a "warm" individual. "She was a good mother. But ... I hate to say this... I loved her, she was responsible, she was good ... But I don't have this feeling which I have towards my father, as something incredibly deep. When I was little, from the beginning, he read the funny papers to me, told me stories, drew pictures, went for walks. Telling stories and walking and stopping and looking; nothing terribly important. But that has been so important in my life. The older I get, the more I know that's so true."

Carle was 10 when his father left for the army and 18 when he came back, "this sick man. Psychologically, physically devastated." Did he try to coax him back to his old self? "We didn't talk any more much. Only superficial things. I had other interests. I was in art school, an artist. I was interested in women." He puffs out his chest in parody of his 18-year-old self. "This old man, coming from Russia, we got along without you." That's pretty much how it felt, he says. "It's so sad."

Looking back, Carle identifies several key figures in his early life whom he likens to "dabs of paint", the spots of colour that enabled his later success. One was his loving foster mother, to whom he was evacuated at 14 and allocated by default. As he stood with his classmates at the station, clutching bits of paper with the names and occupations of their new families, his friend Herman began to whine that Carle had drawn the best family - a baker's family, with the promise of fresh bread. He whined so much that Carle swapped addresses with him. Poor Herman. The baker put him in an unheated room under the eaves and didn't give him extra bread at all. Carle got the nicest family in town. Even now he dreams of them, of how kind they were, and occasionally, in nightmare form, that "I have overstayed my welcome". Carle and his wife went to visit the children of the family last year in Germany, and after bidding them farewell, drove one block, stopped the car and burst into tears.

When Carle was 15, the German government conscripted him and other boys of his age to dig trenches on the Siegfried line. He doesn't care to think about it too deeply, although his wife, he says, thinks he still suffers from post-traumatic stress. "You know about the Siegfried line? To dig trenches. Fifteen. And the first day three people were killed a few feet away. Not children - Russian prisoners or something. The nurses came and started crying. And in Stuttgart, our home town, our house was the only one standing. When I say standing, I mean the roof and windows are gone, and the doors. And ... well, there you are."

Growing up, Carle was always homesick for America. Didn't the allies' bombing of his home damage his affection for the US? "No. No. No. We bombed them, they bombed us. In Germany there has always been this admiration for the English and the Americans. The French were no good, the Italians were no good, the Poles were no good. But the English and the Americans! You had this respect while they bombed you." He laughs. "But the English are not so crazy about the Germans."

The other seminal figure in his childhood was Herr Kraus, his high school art teacher, who at great personal risk noticed Carle's talent and invited him to his house to look at banned art; in Carle's memory, it was Klee, Matisse and Picasso, but he can't be sure. Anyway, it was expressionist and it made a profound impression on him. "I didn't have the slightest idea that something like that existed, because I was used to art being flag-waving, gun-toting Aryans - super-realistic Aryan farmers, the women with their brute arms. That was art. Of course. That was a shock."

After graduating from design school, Carle moved to New York, taking with him a portfolio from his first job as a poster designer: strong, clean work influenced by the Bauhaus. He could, he thinks, have become a fine artist and done very well. (A successful exhibition of his fine art recently closed in Japan and his son - "I have a very nice son - he lives in Brooklyn" - talks about finding a gallery in New York. "Which is fine by me, but I've no illusions. It's not important." He pauses. "If the Museum of Modern Art came and asked me to exhibit, of course, I'd love it! I'm not that modest.")

Carle liked the discipline of commercial art. He worked in advertising in its most glamorous era, the 50s, travelled the world as an international art director, got married, had two children, divorced, and met Bobbie, a warm and funny special needs teacher from North Carolina, who makes affectionate fun of her husband. When they were young, his children had no real idea of his fame, nor were they interested. "They had no respect," he says, smiling. "My kids would come visiting for weekends and I'd say, 'Oh look, I did a book!' They'd say, 'Hey nice, what's on television?' "

When his friend Bill Martin saw a lobster Carle had painted for an advert for antihistamine tablets, he asked if he would illustrate a children's book called Brown Bear, Brown Bear, What Do You See? Something in Carle clicked. "I couldn't describe exactly what it was, but it was something that set me on fire." Bobbie says his success came so incrementally that he has never thought of himself as a star. "He's not dazzled by himself," she says. "He cares deeply about his talent, but not about his success." Much of the profit from his book sales goes into the museum.

The Very Hungry Caterpillar was initially conceived of by Carle as a hungry bookworm, eating its way through the pages. His long-time editor, Ann Beneduce, wasn't sure about a worm as a central character and suggested a caterpillar. "Butterfly!" exclaimed Carle, and so it was born. The book's success has spawned a lot of crank interpretations. It has been described as an allegory of both Christianity and capitalism. "Right after the Wall fell, I was signing books in the former East Germany and was invited by a group of young librarians to have lunch with them. One said the caterpillar is capitalist, he eats into every food one little bit and then the food rots away. Wasteful capitalist. Interesting. I think that if you're indoctrinated, that's how you will see it."

A lot of children's books, he thinks, are messy and noisy, throwing everything at the child, without thought or finesse. He likens his perfectionism to his grandfather's work assembling car engines. "Beautiful parts for Porsche cars. And I'm just doing my very best. A lot of people compromise, knock out a kiddy book. But for me it's my entire life. That's my life. I'm very serious about it."

He has no great sense of competition with his peers. He admires Raymond Briggs for his intricacy of detail. The production company that so successfully filmed The Snowman made a TV version of The Very Hungry Caterpillar, which, says Carle, was "awful. Godawful. I'm ashamed of it." And when he first saw the Dr Seuss books, "I didn't think very highly of them. I thought they were icky illustrations. But I respect him, although it's taken me years and years."

Carle's books are built to last. To his delight, the day of our meeting a photo runs on most US news sites of Michelle Obama reading a copy of Brown Bear to a group of children. It can take years to do a book, or it can happen very quickly. There is no method, he says. He just waits for ideas to develop. On his desk is a scrap of paper with a jotted-down notion: "Books love you." Carle doesn't know if he'll do anything with it yet, but the thought appealed. He smiles. "That's how it begins."