Six hours later she is strangled with a length of cord at her home in west London.
Knuckle-headed police think she was the victim of a burglary as jewellery and her laptop had been taken.
This being a Anthony Horowitz mystery, you know more lurks behind Mrs Cowper’s untimely death.
Cleverly, the author plays himself in the story, explaining that he was approached by former Scotland Yard detective inspector, Daniel Hawthorne, to chronicle his brilliance in solving the crime.
Hawthorne had been an adviser on Horowitz’s popular Foyle’s War television series.
The author respected his sound knowledge of police procedures but found him a pain to deal with.
So within just a few pages the style of the book is laid out with Horowitz shadowing Hawthorne, who is called in by the Yard as a freelance to see if he can achieve a breakthrough in solving Diana’s murder.
Brusque, efficient and uncompromising in his questioning of potential suspects, Hawthorne is the perfect foil to the writer’s more cultured approach to the grisly business of solving murders.
Soon we learn that Diana has an actor son who has taken Hollywood by storm but puts his film commitments on hold while he returns to London to bury his mother and untangle her complex past.
All the writer’s considerable talents are fully exploited as the mesmerising plot unfolds.
And the relationship between the writer and Hawthorne is almost as interesting as the plot with the dialogue between the two thoroughly believable.
Describing Hawthorne, he writes: “He had the same silken quality as a panther or leopard, and there was a strange malevolence in his eyes – they were a soft brown – they seemed to challenge, even to threaten, me.”
For those thinking the master story teller would dominate the narrative, the reverse is true as Horowitz is constantly outsmarted by Hawthorne’s ability to winkle out clues.
The plot develops beautifully when we discover there was a dark chapter in Diana’s long forgotten past, involving the death of a child which may explain why she was targeted by the strangler.
Half the pleasure of reading the book is trying to work out who will join the dots first, Hawthorne or Horowitz.
Although skilfully crafted, there is one flaw in the story.
Hawthorne was fired from the Yard for poor conduct, an issue which would, in these politically correct times, make him persona non grata and certainly exclude him from acting as a consultant on murder cases.
That said The Word Is Murder is splendidly entertaining, absorbing and difficult to put down.
Hawthorne is a well thought out, intriguing character who might yet appear in another book, you sense. That’s if Horowitz doesn’t find him too annoyingly clever.