Author and broadcaster Lynne Truss is as amazed as the rest of us at the phenomenal success of her book -- the rank outsider in the pre-Christmas rush which romped home as a bestseller, selling half a million copies to date. And what is its subject? Punctuation. I asked Truss whether the success of the book had hit home yet.

"Only in the last few weeks have I been able to get any perspective on it at all. Even when you say 'huge bestseller' I think 'no, that didn't really happen.' It's something you imagine someone saying to you someday. It's not real."

Truss has already published three novels, as well as writing many plays for radio. So why a book on punctuation and grammar, rather than another novel?

"I hadn't had a book out for five years and wasn't sure whether I wanted to do books any more or whether I should stick with scriptwriting. I thought it would be a gentle and quiet introduction back into the publishing world, so that when I wanted to publish another novel I could say 'I had a little book on punctuation last year and it sold 10,000 copies' or whatever."

Some 500,000 copies later, it is even on sale in Tesco. In the book, Truss emphasises that she sees herself as a guerilla working in a minority group who care passionately about the disintegration of punctuation and literacy over the last 50 years. She now knows she is not alone. Is this reassuring to her?

"I think I had to be extremely brave to write this book. Writing about sticklerdom and pedants, your intended audience comprises exactly the sort of people who will have a go at you over any little mistake you make. I thought there would be a few sub-editors who would review the book and say 'she's got no right to say this' and it turns out there's half a million of them out there who want to correct me."

Many critics have exposed the extent of their pedantry by their humourless nit-picking at what is, essentially, a very funny book. Even the fan letters have nearly always ended by pointing some small mistake or quibble with Truss's arguments. Has this upset her?

"In a way, yes. The spirit of the book is so anti that sort of thing. I tried very hard not to write it that way."

Truss worked on the Times Higher Education Supplement as a sub-editor, moving on to the Listener magazine as literary editor and then columnist. In 1990, she decided to become a full-time writer and landed a contract with the new Independent newspaper to write about books and arts. She then went to work for The Times for some years, writing novels and radio dramas at the same time.

"Until I was in my mid-30s, I thought I couldn't be a writer. I thought that writing was something you were born with, that you had to have some sort of certificate or some sort of entitlement, and I didn't have it. It took a big burst of confidence and nerve; it seemed like a huge thing to overcome to become a writer. Once I turned the tap on, it has been extraordinary how much has come out."

So how did Truss get from novels and radio dramas to a book on punctuation?

"I really love writing. I was doing radio and decided to spend three years writing and presenting radio -- I thought I would try and concentrate on that and see how I got on -- and I absolutely loved it. Someone asked me to present the series of Cutting a Dash, the radio programme that inspired the book, and it all went from there."

The audience response to the radio series was very positive, but the book did not flow easily from that.

"It took me a while to think 'Is this a subject?' Although there are lots of grammar books for foreigners, no one had really thought of it as a subject for a proper book. It was as I was writing it that I was really aware no one had written it before."

It is tricky to spot a gap in the market and Truss credits the producers of Cutting a Dash, who had the original idea for the programme. Researching Eats, Shoots and Leaves, she uncovered more and more detail, both practical information and deliciously entertaining anecdotes, which could be presented in a small volume.

I asked Truss if her despair over the state of our grammar, which she cites in Eats, Shoots and Leaves as the original inspiration for the book, improved during her research?

"No, it's much worse now. I hadn't realised how bad things were." In the run-up to Valentine's Day, she noticed it with an apostrophe just once. I don't know if I am having an effect or not, but I have noticed that other things, such as the television programme Footballers' Wives, have acquired an apostrophe."

With Eats, Shoots and Leaves still healthily in the bestseller lists and showing no signs of decline, there's still hope for us all.

Eats, Shoots and Leaves is published by Profile, £9.99. Lynne Truss will be appearing at the Oxford Literary Festival from March 23-28.