The brother of the boy whose brutal death inspired Agatha Christie to write The Mousetrap has landed a book deal to tell his own story.

The plot of Christie's theatre play – now, in its 59th year, the longest-running in the world – sees police detective Sergeant Trotter arrive at a guest house on skis to inform the snowed-in residents that a killer may be on their way. A woman who abused children in a nearby house – one of whom died before the courts removed the other children – has recently been murdered, and a note left on her body implied further deaths related to the child abuse would follow.

Terence O'Neill was 10 years old when he testified in court against his foster parents Reginald and Esther Gough, who were accused of the manslaughter of his brother Dennis. The pair had been through many foster homes before they ended up on the Goughs' Shropshire farm in 1945, where they were beaten regularly until one morning, Terence couldn't wake his brother up. A post-mortem examination found that Dennis had been starved for months and weighed only four stone.

The public outcry over the brothers' treatment led to the first provisions to protect other vulnerable children from neglect and cruelty, and provided the basis for Christie's radio play Three Blind Mice, which later became the theatre play The Mousetrap.

Last year, O'Neill – who is now in his 70s – decided to tell the story of his experiences himself, posting his writing on HarperCollins's online slush pile Authonomy, which gives unpublished writers the opportunity to show their work to an online community of readers. His writing was spotted by HarperCollins editors Carole Tonkinson and Kate Latham, and is set for publication as Someone to Love Us next month.

"We're very excited to be publishing it," said Laura Summers at HarperCollins. "We have published a few novels picked up on Authonomy which have gone on to be bestsellers, and we hope the same thing will happen here. Terry the author is so thrilled it's being published."