At some point in the life of every book-loving child you run out of road. There’s a reluctance to leave beloved books behind, but the urge to be grown-up is too intoxicating. Nancy Drew and the Famous Five are discreetly shoved to the back of the wardrobe and there you stand at a literary crossroads. Where next?

For me, between ditching the Secret Seven and moving on to The Pearl, there was Agatha Christie, whom I discovered during a period of immobility. Christie was a catalyst who walked me over the bridge (albeit on crutches) and into my adult life of reading.

Bedbound, the days dragged in the children’s ward. So I consumed the stack of secondhand paperbacks I’d brought. The covers are still vivid: peacock feathers (Third Girl), a blackbird skeleton (A Pocketful of Rye) and a strange apple/skull hybrid (Halloween Party).

When I opened one, gone were the hospital bed and cellular blanket: I was off on a boat on the River Nile, or in sweltering Mesopotamia, or shadowing someone down a murky London side street.

People now scoff at Christie’s books as formulaic, especially when contemporary crime writing is a very different beast. But I was hooked. I’d scour secondhand shops and garden fetes, buying any editions I could find. By 13 I’d read every one of her books.

Agatha Mary Clarissa Miller was born 125 years ago, on September 15th, 1890. She began writing mysteries after a dare from her sister. Christie’s books have sold two billion copies worldwide, and she is the only woman who has ever had three plays running simultaneously in the West End. Last month 10 lost plays were discovered, a gift for hopeful fans.

It’s staggering to think that Christie was turned down by several publishers, and it took her five years to get The Mysterious Affair at Styles published. (She was paid £25.) We appropriate her as a writer, but Christie had other lives: she was an accomplished musician and soprano, occasionally surfed and qualified as a dispenser, which is how she acquired such a handy knowledge of poisons.

Millions like me devoured the books, and interest in Christie’s personal life reached a frenzy when she disappeared for 11 days in 1926. Amid rumours of marital strife (and her husband’s cheating), she vanished.

To commemorate the 125th anniversary, the author Kate Mosse, who founded the Bailey’s Prize for Women’s Fiction, has written Eleven Days, a short story about Christie’s own metamystery; she explains why Christie endures: “Because she knows human nature. All her plots are puzzles – ingenious, nifty, plausible (mostly!), where an individual person’s behaviour, their character, is the direct cause of the story that unravels. Christie’s focus is on psychology, understandable human failings, rather than violence or guts and gore for the sake of it, and there’s always a satisfying, coherent resolution, a proper full stop.”

The screen adaptations continue to be a staple of weekend TV; Peter Ustinov was a particularly superb Poirot. And they still endure: after the BBC’s recent Tommy & Tuppence series, starring David Walliams and Jessica Raine as the spouse detective, there is to be a new adaptation of And Then There Were None (Christie’s biggest-selling novel), starring Aidan Turner.

My bookshelves have expanded hugely over the years: a paper beast of many spines. Glancing up now, I see the Fontana paperbacks, faux leather editions and green vintage Penguins. They have survived moves and many culls, so this might be the year to start rereading them.

On Tuesday, crime writers give their verdict on Agatha Christie at irishtimes.com/books and Kate Mosse reads Eleven Days at bbc.co.uk/culture; 125 Years of Agatha Christie is at the Theatre Royal, Waterford, on September 25th