Catherine O'Flynn: contender for the year's grandest prizes

A Birmingham shop worker who captured the moods of the mall

For the benefit of the tape, I am holding up my heirloom copy of The KnowHow Book of Spycraft which the interviewee has immediately recognised. "Excellent," Catherine O'Flynn laughs, her slender figure silhouetted against the window of her neat, suburban Birmingham terrace, as we flick through the pages of secret disguises and home-made codes. "It's not actually the one I used to have, but it looks very similar, with illustrations by the same guy. I loved the whole detection series. Mine was called Clues and Suspects and there's another called Fakes and Forgeries."

Since Birmingham's tiny yet illustrious Tindal Street Press published it earlier this year, O'Flynn's gripping debut novel What Was Lost (8.99) has been Radio 5 Live's Book of the Month, and long- or short-listed for four major awards. Most recently, it has reached the shortlist for the Costa First Novel Award, to be announced early in January. At the novel's heart is the engaging character of 10-year-old Kate Meaney, a fiercely dedicated amateur sleuth. Modelling her bedroom and habits on tips in How to be a Detective, a cherished gift from her dad, Kate regularly slopes off to the local Green Oaks shopping centre to file surveillance notes in her top-secret Detective Notebook.

Sitting in Vanezi's caf with her ever-alert sidekick, Mickey the Monkey, Kate jots down her suspicions: "No Mr Tan today, but instead a woman with a suspiciously bad wig. Are they connected???... Man seen eating orange peel from brown paper bag. Followed him for 40 minutes but no further deviance observed."

Given how nuanced Kate's observations are, were such games part of O'Flynn's childhood? "Very much so," she emphatically agrees. "I thought I was a junior detective. I was a lot less professional about it than Kate is, and a lot more half-hearted though I did try and set up my bedroom as they advise you to in my version of the book. When I was writing What Was Lost, I looked back at those detective books, and they included ideas like following strange men down dark alleyways at night completely insane!

"And I thought these things were like textbooks!" she exclaims, adding that her own freedom to roam was slightly more modest than Kate's ability to stake out the shopping centre. "Really, the extent of my detection went as far as a bank near where I lived. This bank was a gleaming, Sixties building, which was really out of character for Nechells, the run-down and tatty part of inner-city Birmingham where I grew up. I used to go and hang out there quite a bit, because I was convinced that this was a centre for international crime. I'd take down car registration numbers. Obviously, nothing ever happened at all."

For Kate, however, something does happen. One day, she simply vanishes without trace. For little more than its first quarter, What Was Lost follows Kate's perky, resourceful, brightly imagined character, eating tea in front of The Rockford Files and fantasising about proper stationery and bus adverts for her agency, before she disappears after reluctantly attending a grammar-school entrance exam. Police are baffled. Her unpatronising adult friend, Adrian, the neighbouring sweet-shop owner's son who is struggling post-degree to settle on a career, becomes a suspect and is himself soon hounded into disappearing.

Twenty years later, Adrian's sister Lisa, stalled in a dead-end retail job in Green Oaks shopping centre, teams up with a security guard who thinks he has seen a little girl at night on the centre's CCTV. Intrigued, and a bit spooked, the two begin scouring the centre for any trace of the trapped girl, who in some way resembles the little missing detective of 20 years earlier. As a debut novel, What Was Lost is startlingly assured, layering a mystery with Murder on the Orient Express-undertones of collective guilt onto a subtle exploration of disenchantment. The combination of Kate's early purposefulness and the listless disillusionment of Lisa and Kurt generates a peculiar disjuncture that accentuates the elegiac sense of loss.

Kate's mysterious absence haunts the poignant, evocative and (given the tragic subject matter) surprisingly funny remainder of What Was Lost, but the whole book is framed by a pervasive sense of nostalgia. O'Flynn readily admits that there is much of her childhood in Kate, but did she find it difficult to recall a 10-year-old's thoughts? "The short answer is: no, it was almost alarmingly easy to slip into that character. It's some kind of comment on myself that I find that's almost my default voice. Without being too psychoanalytical, I was always the baby of our family, which has in some ways stuck with me."

In her book, Kate is in a lonely position that strikes a chord with O'Flynn's own experience. As the youngest by a decade of six siblings, she was content but "sort of quite lonely", and left to her own devices. "Nobody was paying me a blind bit of notice," she recollects.

Beyond a wistfulness for the gumshoe obsession of Kate's truncated childhood, there's also a lament for the lost physical heritage of O'Flynn's native stamping ground. "The landscape that Kate grows up in and that Kurt, the security guard, spent a lot of his childhood playing in is the same one that I grew up in," she confirms. "It had been industrial and would later go on to retail and leisure use, but when I was growing up in the 1970s all the factories had pretty much closed down, leaving loads of places to go and play in this wasteland of concrete plateaux and canals. It was very evocative of what was lost," she says. "As a child, I loved playing in those kinds of places. I'm really kind of nostalgic for them now."

With good reason: her old haunts have disappeared under Star City, a casino and leisure complex that has "devastated" her memories of the suburb. "Subconsciously, I find it incredibly traumatic," O'Flynn says. "It's as though a spaceship has landed in the middle of the neighbourhood."

Among many other jobs, O'Flynn has, like Lisa, worked in a mall record store. This brought her into direct contact with varied people "who had lost their way a bit", from aimless shoppers to people like Kurt, whose brief tenure drifts into 13 years, and Lisa, whose entire life seems superficial. Unlike these characters, O'Flynn mostly enjoyed her decade in retail, "just doing what a lot of people would consider low-prestige jobs". She has certainly put her years behind the till to excellent use.

"There's a whole world there going on behind the scenes," she says. "Often, when you're in these jobs, the thing that keeps you there is the camaraderie, because you all band together and despise your manager. It's depressing being among people day after day who are in jobs they don't like, but there was always the hilarity." Several acerbic shop-worker rants, delivered pitch-perfect by O'Flynn's well-tuned ear, remind me of the horror of my own years of working in Dillons bookstores, and give a brittle, exuberant contrast to the bland white noise of the shopping centre.

White noise was the book's genesis. Scribbling her experiences down in the evenings, O'Flynn wanted to capture the cacophony of voices in the shopping centre, which gave a context to Kate's adventuring. "I never thought I'd be capable of writing a novel. I thought the whole idea was just ridiculous, really, but then this thing grew and developed from my notes until gradually I thought this could be a novel. I always kept my expectations very low, and I still do go through phases where I'm not at all career-oriented or ambitious."

With such low expectations, how has she coped with this year's acclaim, which includes long-listing for the Man Booker and Orange prizes and short-listing for the Guardian First Book, as well as the Costa, award? "It was really lovely," she says simply, but her gratitude is not without reservations. "Because I was actively managing to avoid getting incredibly hopeful, to some extent that insulated me from the good news as well." Avoiding a rush into a second novel, O'Flynn is weathering a "surreal, slightly paranoid, 'No, this probably isn't really true'" frame of mind. The prize listings are good for motivation, she concedes. But O'Flynn thinks it could be another six months before she can look back, with or without nostalgia, and think: yes, that was a remarkable year.

Biography: Catherine O'Flynn

The youngest of six, Catherine O'Flynn was born in Birmingham in 1970 to Irish parents; her mum, a teacher, died when she was 15, and her dad, a newsagent, when she was 23. After King Edward's Grammar School, she studied anthropology and sociology at Manchester University. Her first job was in journalism, writing for listings papers. A series of shopping-centre jobs followed. Her only "proper" job was as web editor for a computing publisher. She has been a postwoman (briefly) and a mystery shopper (very briefly), and currently works in a box office when she isn't writing. What Was Lost, long-listed for the Man Booker Prize and now short-listed for the Costa First Novel Award, is published by Tindal Street Press