Sitting in the QI bookshop in Turl Street, Oxford, listening to owners John Lloyd and John Mitchinson exchange quick-fire information about bananas, it is easy to understand the success of the TV show which they write - QI, which stands for Quite Interesting.

"Did you know the banana is not a tree? It's a giant herb, really," says Mitchinson. Lloyd chips in: "And then you start to ask more questions and you discover that a banana is technically a berry, not a fruit. And then you find out that all the bananas that we know about haven't been grown from seeds.

"They come from bananas that haven't had sex for thousands of years, so banana plantations are being devastated by fungal diseases."

Michinson says: "Of course, we all know that bananas come from the Third World - except that they are grown in polytunnels in Iceland.

"Actually, bananas come from Ireland, because Ffyffes is an Irish company. And, of course, Panama hats actually come from Ecuador."

The pair met when they were neighbours in the North Oxfordshire village of Great Tew, working long hours and commuting to London. Lloyd said: "I had a bit of a midlife crisis. I had become a radio producer ridiculously young, after joining the Footlights comedy revue in Cambridge, and I worked ridiculously hard.

"I was doing commercials, travelling all over the world and I felt that I had done everything that I could in my career. I lost faith in what I was doing and got very stuck. While I was travelling I read lots of books and gradually realised that everything you know turns out not to be true.

"For instance, how many moons does the earth have? I read somewhere that a second moon had been discovered. I started to collect facts."

At the time, Mitchinson was head of publisher Cassell, specialising in illustrated reference books and acting as Deputy Publisher of the Orion Group, now the UK's third largest publisher. Before that, he had spent six years as marketing director of bookseller Waterstone's.

Lloyd said: "We had a very small cottage in Great Tew. It was tiny, and we had two children, but the house was full of books and his house was the same. We were in the pub - John was publishing reference books at the time - and I had an idea for a different kind of reference book.

"We wanted to provide stimulating facts, but humour is important as well. We try to tackle the big questions as well as how big a barnacle's willy is."

They believe that information is dendritic - tree-shaped - with many connections, and always growing. Mitchinson said: "A lot of it came from kid's questions - why is the sky blue?"

In the end, the TV show came before the book, when BBC2 ordered 16 shows in 2002. "We tried to sell it to BBC 1," said Lloyd, who was responsible for Blackadder and Spitting Image. "But they said it was too clever."

Now, as filming starts for the fifth series, and the DVD hits the shops, the pair have finally produced the reference book they planned all those years ago in the Falkland Arms in Great Tew. Called The Book of General Ignorance, it includes quuestions and answers exploding "common misconceptions", including the belief that Henry VIII had six wives, or that Mount Everest is the world's tallest mountain.

In the years between the idea and the book, as well as creating a successful TV show, the pair have also opened the QI club and bookshop, designed as "a place where you can have a decent conversation", says Lloyd. He says the club is not exclusive or elitist, but then adds: "We like to make sure that people will fit in with the other members."

The bookshop is also ideosyncratic, with the books shelved not in the usual categories of fiction, biography, etc, but in themes such as greed and power. So 'the big picture' has Milton's Paradise Lost alongside modern scientist Richard Dawkins, 'landscape' encompasses novelists such as Thomas Hardy, and a 'good life' section carries guides to growing your own vegetables alongside Zadie Smith's On Beauty.

They say the bookshop and club "just about" pay for themselves, while the TV programme-making company, run from the same building, has created cult watching, helped by chairman Stephen Fry and team member Alan Davies.

The Book of General Ignorance is published by Faber at £12.99. The sixth series of QI is on BBC2 on Fridays at 10pm.