What happens when two writers find romance - after they already have eight children and three grandchildren between them? William Horwood and Helen Rappaport met online through Dating Direct nearly three years ago. Within two months of their first meeting over tea and scones at the Old Parsonage in Oxford, they knew things were serious, that they wanted to propagate. From such fertile beginnings, Dark Hearts of Chicago was born.

A historical thriller set in Chicago during the 1893 World Fair, the action takes place over 11 days. Young, ambitious reporter Emily Strauss tricks her way into newspaper magnate Joseph Pulitzer's office and persuades him to send her on an assignment: to find out what happened to a young Latvian woman, Anna Zemeckis, whose body has allegedly been found in Chicago. Only Anna is not dead. Pulled beaten and bloodied from the horrendously polluted Bubbly Creek, she's been carted off to the local insane asylum, where she's in great danger of being lobotomised.

Literary it ain't, but Dark Hearts is such fun to read, with two feisty female leads and a punchy, gutsy story that makes magnificent use of its brutal, corrupt, grandiose backdrop. Rooted in extensive research, sometimes it's difficult to assess where the lines of fact and fiction blur.

"The historical context, the central events are correct and the details are correct, but the characters are fiction, apart from one or two key characters," William explained. Even so, all the characters are born of research. Emily, for example, is a composite of three female reporters from that time.

William has written 16 novels, including the Duncton Wood series, several sequels to Kenneth Grahame's The Wind in the Willows and - most recently - a fictionalised memoir, The Boy with No Shoes. Meanwhile, Helen is a 19th-century historian and expert on the black Crimean nurse Mary Seacole. She has just published an account of women in the Crimean War, No Place For Ladies. So given their very different areas of expertise, how did the novel come about?

William said: "With a mutual need to do something together, rather than a book idea." She added: "Initially, we thought of doing a contemporary Russian project. There would have been an element of my Russian knowledge, with William's knowledge of London's Fleet Street." But that idea had been pre-empted, so they took a list of six more to their agent, who liked best the idea of a historical thriller set in Chicago. Two research trips later, they had the plot and characters in place and over the next nine months the book was brought to term.

Helen is in her late fifties, William a little older. He said: "It's the maturity that comes from two older people and life experience that made it possible for us to write the book."

She said: "Assimilated knowledge, too. There's a huge amount in both of us that's come through into the book without us probably being able to realise it." They do that a lot; finish each other's sentences. So how did they actually write the book?

At the time, William lived above Helen in a flat on Woodstock Road, although he's since bought a house nearby. He wrote the first draft and after completing a few thousand words would email it for comment to Helen. "At the beginning, I was a bit timid about cutting and rewriting William's sacred prose," Helen said.

He cut in: "She thought I was a genius and then realised I wasn't." However as the book moved on and the story developed a momentum, she became more confident and the writing more collaborative.

They both feel that working together, as well as being enormous fun, has strengthened their own writing. "It's taught me to look differently at how I write history and to have a greater sense of narrative," Helen said. Meanwhile, William thinks he writes much tighter than before and his research skills are much improved.

There were clashes. "I would get on to William about violence and he would go on about my repeated use of the word And at the beginning of sentences." Interestingly, it was Helen who cut the sentiment. He said: "It's a deeply dispiriting thing to sit weeping over your typewriter and half an hour later to have a hardnosed modern woman slash the very things that have made you weep."

Both are looking forward to their slot at the Oxford Literary Festival. He said: "Readers have paid money to come and see us, so we intend to move them, shake them, change them, make them excited. It's going to be illustrated, dynamic, highly interactive.

"We want to get away from this ridiculous mystique that writers are some kind of royalty," Helen said. "Our primary purpose is as communicators." So don't expect Queen Helen or King William to take the stage on Thursday. Instead, there will be two mature entertainers keen to explain the pain and ecstasy of birthing a book together.