Writing Twenties Girl was like going on my own magical mystery tour. My U.S. editor Susan Kamil had once said casually, "You should write a ghost story one day." This comment stayed with me for years. I loved the idea, but didn't know who my ghost could be. I've always loved the glamour and spirit of the 1920s, and the idea came to me of a flapper ghost. A feisty, fun, glamorous girl who adored to dance and drink cocktails and get her own way. I wanted her to be a determined character who would blast into the life of someone with no warning and cause havoc. I then decided she should haunt a thoroughly modern girl, with all the culture clashes and comedy that would bring.

Having come up with this idea I loved it, so it then remained to plunge myself into 1920s research, which was no hardship at all, as I find the era fascinating. I researched vintage make-up, vintage dresses, read fiction from the period, investigated 1920s slang, and tried to channel as much I could of those feisty flappers who cut their hair short (shock!), smoked cigarettes in public (shock!), had sex (shock!) and generally rebelled in all the outrageous ways they could.

The book isn't a period piece though. It's a modern story about two girls and their sparky friendship, right here in the 21st century. One of them just happens to be a ghost from the 1920s. It's a quest, a romance, and a coming of age... and above all a comedy. It's no exaggeration to say that writing the character of Sadie made me look at life differently, and I hope some readers feel the same way. —Sophie Kinsella