We're entering a room in London's West End that is a shrine to a skinny, self-absorbed 12-year-old boy, who is bald but for three tufts of hair, with a round face and no eyebrows, and who is nobody's idea of a superstar.

"Wow!" exclaims Jeff Kinney, creator of this nerdy kid whose diaries are stacked on every shelf, while cartoons of him and scads of merchandising material are displayed everywhere in the Strand headquarters of his British publishers.

It's difficult to fathom why the bestselling American author and illustrator should be surprised. Nonetheless, the 41-year-old father of two looks bemused, baffled and bewildered that his gauche cartoon character should merit such adulatory treatment. After all, as Kinney is quick to point out, the angst-ridden tweenager, Greg Heffley, is an unlikely hero.

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Greg is the "flawed" protagonist and "unreliable narrator" – Kinney's description – of the publishing phenomenon that is the Diaries Of A Wimpy Kid, which have sold more than 75 million copies in 41 languages in 44 countries. In March, the diaries were voted best children's book of the last decade, beating Harry Potter.

There are three major movie versions, which have collectively grossed more than £125 million, and Kinney is working on the script for an animated TV special. Last year, the sixth book of the series, Diary Of A Wimpy Kid: Cabin Fever, was this country's biggest and fastest-selling children's book – in the first week of publication one was bought every four seconds. On a global author tour, Kinney signed more than 17,000 copies for legions of young fans, he reports, gazing at his right hand in disbelief that he still has use of it. Although, he adds, his eyesight is definitely deteriorating.

Doubtless that signing record will be beaten any day now, because book seven, Diary Of A Wimpy Kid: The Third Wheel, has just been published. It doesn't disappoint; it's even more laugh-out-loud than the previous half-dozen as Greg struggles with the fact his delinquent elder brother, Rodrick, is becoming almost too cool for school, while baby bro' Manny is peskier than ever and best friend Rowley's loyalty is in doubt.

Meanwhile, the Valentine's Day dance looms at Greg's middle school – also known as junior high, it's the two-year bridge from elementary school to high school in the American education system, a kind of no-teen's land between childhood and adolescence when, as Kinney notes, kids are still in "the larval state". In The Third Wheel, Greg is actually in the foetal state as he regales us – Tristram Shandy style – with recollections of "stuff that happened to me even BEFORE I was born" while swimming around in the dark, doing backflips and taking naps whenever he wanted inside his mother's belly.

A risky move but hopefully an amusing one, ventures Kinney, who never kept a diary until he was in his late twenties, although it was filled with cartoons and eventually morphed into his books. "I wish I had kept a journal," sighs this modest, open-faced "ordinary guy" with the crooked grin and genial manner.

He's just flown in from the Plainville (population: 8000), Massachusetts home he shares with wife Julie and their two sons, Will, 10, and Grant, seven, who have met three US presidents and hung out on film sets thanks to their father's journey from failed newspaper cartoonist to being voted one of Time magazine's Top 100 Most Influential People in 2009. ("I'm not even the most influential person in my own house," Kinney remarked at the time.)

Invited to speak at the Davos World Economic Forum, where he shared the stage with James Cameron, the director of Titanic and Avatar, he reportedly thought he was "the light entertainment" for the great and the good since he felt he didn't belong, but confirms it was "a good experience". Sometimes, Kinney – one of four children of a military analyst for the Pentagon and a teacher who later did a doctorate in childhood education – feels as if he's watching all this happening to someone else. "It's a bit like I'm on The Truman Show," he's forever telling interviewers – words he repeats to me. "It's surreal; it just gets stranger every day."

All these plaudits may still mystify the Maryland-born and Washington, DC-raised author, who still works at the day job he loves, with Poptropica.com, the world's biggest educational gaming site for children, which he devised. He acknowledges, however, that he's delighted he's got shy, pre-teen boys reading – although he insists that he never set out to do so. He simply wanted to entertain and amuse. In fact, the Diaries Of A Wimpy Kid – which have been described as Seinfeld for children – were never intended for young readers. "I honestly thought I was writing a nostalgic book about an ordinary childhood like my own for adults, which had maybe some parallels with JD Salinger's The Catcher In The Rye – Greg being, unconsciously, a sort of Holden Caulfield and me having been a bit of a wimpy kid myself, skinny and scared sometimes, a bit awkward and anxious," he confesses. "But I do think if there's a message in the diaries it's that reading can be and should be fun. I just hope kids get a kick out of the books and laugh at the jokes – because I'm just a gag writer. I get lots of ideas back home in Plainville, which is a small town, where I'm a cub master in the Scouts and I coach my kids's sports teams.

"The downside of the success of Wimpy Kid is that I don't get to spend enough time with my family, but it has given us many opportunities, although we still live in the same house, drive the same car, wear pretty much the same clothes. We've helped out our families and made some charitable donations, and we bought the house next door so I'd have a quiet place to draw and write. I work on the books at weekends – they're my hobby. Also, we're helping our community. We've bought a dilapidated building in Plainville which we're going to improve so the town centre looks nicer. We're definitely having fun. I believe the diaries are good, wholesome books that I'm certainly not writing to impress literary critics."

What Kinney does do with his books – part comic-strip, part handwritten notes that look like real diaries – is impress children like nine-year-old Evan Lonney, of Edinburgh, who is an obsessive fan. I asked Evan to tell me what he'd ask Jeff Kinney if he were interviewing him, so I'm armed with his foolscap sheet of questions. "Oh, that's so neat!" exclaims Kinney when I show it to him. Over to our cub reporter.

What did you read when you were a kid?

"I loved comics," replies Kinney, who was a gaming geek in his late teens. "I read everything from Donald Duck to the Uncle Scrooge comics for the storytelling. My dad was an avid newspaper comic-strip reader, so when I got older I really liked The Far Side. I also loved Judy Blume's coming-of-age books; then I graduated to fantasy and my all-time favourite is The Hobbit – a great book."

What advice do you have for young writers?

"My advice would be to take an author you like and to copy their style, but to tell a different story because when you write or illustrate in someone else's style you can figure out how they created what they did – that helps you to find your own voice," says Kinney.

What does the future hold for Greg?

"Well, Greg is preserved in amber," he discloses. "His future is his present. He's stuck in pre-adolescence because all the best cartoon characters never grow up anyway. Look at Charlie Brown! He has a first day of school every year – he's also bald like Greg. But in concrete terms, I plan to write at least three more books and Greg will suffer many more trials and tribulations, trapped in a state of arrested development for ever."

As we wind up the interview Kinney signs a copy of his book for Evan, generously embellishing it with cartoons. Then – at my request – he shows me a picture on his mobile phone of him with his family, including his parents, at the White House in May with President Obama, Michelle and daughters Natasha and Malia. "The president told us he gave Diary Of A Wimpy Kid: Cabin Fever to his daughters last Christmas; he also said he's a huge fan of Greg himself."

Who knew? It seems that the leader of the western world is a wimpy kid at heart – just like the rest of us. Meantime, though, 10-year-old Will Kinney has abandoned his father's diaries. He's heavily into Harry Potter.

Diary Of A Wimpy Kid: The Third Wheel by Jeff Kinney is published by Puffin, priced £12.99. Diary Of A Wimpy Kid: Dog Days is out now on DVD and Blu-ray.