IT'S 10 years next month since Geri Halliwell sensationally quit The Spice Girls in the middle of a world tour but when I remind her of this she just says "Is it?" and "Wow" and "How ironic". She was interested enough in the 1990s pop phenomenon to become Ginger Spice again for last year's reunion shows but beyond that I sense that the 10th anniversary of her departure means little.

And why should it? In her candid 2002 autobiography Just For The Record Halliwell characterises her post-Spice Girls life as "a journey into the unknown", but you could just as easily say it was the opposite: a journey into the even-better-known. Cashing in her girl power chips she put a downpayment on an LA sunshine life and, tellingly, is still too famous today to go on any TV programme with "celebrity" in the title.

The latest chapter in her journey takes her where Madonna and Kylie Minogue have gone before. She has written a children's book. Actually she has written six children's books, each containing three stories. They will be published monthly until October.

"I'm a very creative person. I have a big imagination. I love words," she tells me. "During the Spice Girls I was instrumental in writing the lyrics. I loved writing them. Then when we did videos I liked doing the treatments because I liked telling stories. I've always been a storyteller ever since I was a kid and I've always kept diaires. I've got stacks of diaries."

She enjoys writing, she says, because "you get to play God in your own world. I like that. I get to be the puppet master. If you ask anyone who's worked with me, throughout my career with the Spice Girls, I was always very inventive and imaginative. So it just felt like a natural progression."

We are talking in a reception room in her north London home, just her and me and two massive cream sofas. We have a glass of wine each and we are sharing a Twix.

The house is large, but not excessively so. It is busy with people, though. I have already met Halliwell's Aragon-born mother Ana Maria, who fires questions at me in Spanish when I foolishly tell her I am learning the language, and her two-year-old daughter Bluebell, who will tootle in again later and undertake a wobbly lap of the sofa. Bluebell has her hair in bunches and she looks like her mum. She's a cute kid.

But as well as the family there's the "help": a gardener, a couple of personal assistants, and a publicist. Everyone (except the gardener) departed for the kitchen when Halliwell arrived, but through the sliding doors that separate us from them I can hear chatter and, occasionally, the banshee wail of an irate toddler. Halliwell herself is dressed casually in vest and yoga leggings, with no shoes or socks on her feet. Her hair and make-up are already perfect, however, as she has a television appearance scheduled for later in the afternoon.

Her street is affluent but not exclusive-looking, which surprises me. There is an above-average quota of SUVs parked on the kerbside but the only clue to the presence of a celebrity pop star in the neighbourhood is provided by the young men sitting in shabby cars or loitering on the street corner in wet weather gear. They are paparazzi, and they want any shot of Halliwell they can sell.

They've been lucky this week. Over the last few days many of the tabloids have featured pictures of her with a man identified as Ivan Valez. I wonder, idly, if it was him that answered the entry-phone system when I first pressed the buzzer. Unfortunately, his place or otherwise in her life a subject she is unwilling to discuss.

The books, of course, are not. They star one Ugenia Lavender, a spunky nine-year-old girl who wears Ugg boots and has a hair-flick her creator describes as being like Cameron Diaz's in There's Something About Mary. Ugenia's mother, Pandora, works in breakfast television and her father, Edward, is a globe-trotting dinosaur expert (think Indiana Jones-meets-Professor Branestawm). Otherwise, they are a perfectly normal British family.

The Lavenders have spent the last eight years travelling the world but have now returned to a dull life in a dull street in a dull town called Boxmore. Playing supporting roles in the tales are Granny Betty, who dispenses homilies such as "The only thing to fear is fear itself", and a gang of schoolfriends: Rudy, Bronte and Crazy Trevor.

Halliwell is in the middle of telling me how she secured her book deal when she breaks off, salutes and chirrups: "Good morning, Mr Magpie." I freeze, somewhat bamboozled by this surreal and abrupt non sequitur. I realise she has spotted a bird in the garden and is invoking a good luck greeting. It's an amusing interlude, but a bonkers one.

For the record, then, Halliwell began writing the Ugenia stories four years ago in Los Angeles. "I wanted to find another medium for girl power," she says. She also felt that the books currently on the market for eight- and nine-year-olds were too gender-defined. "There was Horrid Henry, which was great but all about blokes, or there was Flower Fairies, which was very pink. There was nothing in between. So I wanted to create a character that was human and well-rounded but who boys and girls could identify with. Ugenia is tomboyish but equally she nicks her mother's nail varnish and she wants to be pretty."

Halliwell showed some early versions to the publishers MacMillan, who encouraged her to write more and then offered her a six book deal. There is also, she says, a TV series "in the pipeline".

Of course the scornful snorts that accompanied Halliwell's appointment as a global ambassador for the United Nations Population Fund in 1999 were nothing to the howls of laughter that have greeted this latest career development. Celeb-watching website Holy Moly ran an article about the books under the headline "Proust shits his pants" and, while the grown-ups who run the broadsheets have used less scatalogical language, their estimation of Halliwell's literary gifts is no different. I ask her if she is prepared for - how can I put it? - a cynical reception.

"I can only talk about my own journey and my own adventure and for me, the number one key that I always ask myself is whether I am being authentic, is this something really true and honest? Because people will smell it otherwise. I really trust the public. I think they make their own decisions and I feel like I've worked my hardest to make a great book and enjoyed every minute of it and I want to share it."

As I am trying to figure out if that's a yes or a no, she continues with her answer. "People might pick up that book because I'm a famous person or they might not," she says. "But if one person picks it up and it gets them reading, then brilliant. For me, it's authentic. I am a reader, I am a writer. I understand that we all feel safe with a certain person doing a certain thing but if you look at most people's lives, they don't stay doing the same job all the way."

Halliwell has also recorded an audio version of the books, spending a considerable number of weeks in the studio dreaming up the different voices. There's a Mr Patel who features in some of the later stories and Halliwell drops into an Indian accent so I can hear what he sounds like. The accent isn't bad but it veers dangerously towards the sort of ill-advised caricature that could land her in trouble with the PC brigade. Not that she worries about things like that - this, after all, is the woman who praised Margaret Thatcher in an interview with The Spectator in 1996.

"I come from Watford, which is multicultural," she explains. "There were Pakistanis on the corner of my road and they were my mates, we'd play in the street together. I think that's part of our lives, a multicultural society."

I tell her that the character of Rudy reminds me of Justin, the gay, fashion-obsessed schoolboy nephew of Betty Suarez in the US TV series Ugly Betty. She squeals with delight. "Absolutely. He's so based on that. That's where I got my inspiration." What does Rudy sound like, then? "He talkth a little bit like thith," she lisps, camping it up. So is Rudy gay? "I would imagine so," she says.

Crazy Trevor, meanwhile, is based partly on Wayne Rooney and partly on someone Halliwell knew in LA, while Ugenia's celebrity chef uncle - did I forgot to mention him? - is a blend of Gordon Ramsay and George Michael, if you can imagine such a fabulous concoction. Ugenia, of course, bears more than a passing resemblance to Halliwell herself. "Definitely there's elements of me in there," she says.

Halliwell wrote the books on a laptop in her upstairs study. She still visits LA from time to time - "It keeps my creativity alive and I like it for that reason. It gives me space to breathe" - but since Bluebell was born in 2006 she has made London her home. It helps that her good friend George Michael, who grew up near Watford himself, is also based in the UK now. "We're like family," she says. "We have a really good connection. He can laugh at himself and I can too."

Like Michael, Halliwell has had her troubles in the past and I sense that their friendship is based on solidarity in extremis as much as on a shared hometown and sense of humour. In Just For The Record Halliwell reveals the extent of an eating disorder that first surfaced in her teens but which worsened after the death of her father in November 1993, and continued for the rest of the decade. A year-and-a-half before the Spice Girls had their first number one she checked into the psychiatric unit of Watford General Hospital and was prescribed Prozac. In November 2000, at Michael's house in LA, she hit rock bottom when she binged on chocolate cake thrown into a rubbish bin after a Thanksgiving dinner. But she sought help and found it.

Today, she seems taller than her five foot one-and-a-half and, though it's hardly scientific, that fact seems to me to indicate rude health. She is certainly confident enough.

"All that stuff is just so behind me," she says when I mention her battles with food. "I feel I've moved on and it's not part of my life any more. My life is thinking about my daughter and trying to look after her and being a single parent. What I like about writing is that it allows me to be at home with my child. I can write but then I can go down and play in the garden with her."

I ask her how she felt when she heard that John Prescott had also suffered from an eating disorder. "I didn't know that," she says. Is she surprised? "No. I think lots of men have it. It's one of those things. Poor love."

Music, too, is behind her. "Never say never, but I feel like I've climbed the mountain of music. I've given it my best. I know some people want to go on and on and on but for me I have a lot internal change going on and I have to honour that. The other thing going on is that I've realised control is just an illusion. The more I try to control things the more out of control they've become. She's taught me that" - and she points at Bluebell, now barrelling into a side-room clutching a beaker of water.

She doesn't regret leaving The Spice Girls though she does regret the manner of her departure. Had she had what she calls "the tools" to deal with the problems she perceived in the band - and in her own life - she might not have left so abruptly. For a start she would have completed the dates on the 1998 tour, including the September homecoming concert at Wembley Stadium. Her relationship with the other band members, particularly Victoria Beckham, is repaired now but still, the 2007 reunion tour felt like a second chance.

"There was a certain amount of pressure," she says. "I felt a responsibility to my four bandmates to be the best that I could possibly be. We had to up our game and I wanted to stretch myself because I knew I might not have this opportunity again, and I wanted to feel proud of what I had achieved. But in a way there was nothing to lose after that. So we just had fun. The best thing we could have done was enjoy it." Which is exactly what she did. "I felt that I was really alive on the stage," she says. "That was liberation."

I've just broached the subject of our new prime minister - "The impression I got of him as a human being was that he seemed genuine and kind and smart", she says - when there's another arrival in the room, one that can walk and talk properly and who doesn't call Halliwell "Mummy". It's the publicist: a car is on its way to take her to the Blue Peter studio. Time's up.

And so, with a hurried goodbye, she departs. Her people show me out. There's a button to push to make the gates work, apparently. I look for it, find it, press it. As the gates swing open and I pass into the suburban street, the paparazzi lenses lift and swing towards me expectantly. I smile and pose a little but the cameras drop without the shutters firing. Perhaps I should have saluted and shouted, "Good morning, Mr Magpie" - or just flicked them the bird like a real celebrity would.

*Ugenia Lavender by Geri Halliwell is out now (Macmillan Children's Books, £6.99) www.ugenialavender.com