Marble Hall Murders by Anthony Horowitz

Susan Ryeland has had enough of murder.

She’s edited two novels about the famous detective, Atticus Pünd, and both times she’s come close to being killed. Now she’s back in England and she’s been persuaded to work on a third.
The new ‘continuation’ novel is by Eliot Crace, grandson of Miriam Crace who was the biggest selling children’s author in the world until her death exactly twenty years ago.
Eliot believes that Miriam was deliberately poisoned. And when he tells Susan that he has hidden the identity of Miriam’s killer inside his book, Susan knows she’s in trouble once again.
As Susan works on Pünd’s Last Case, a story set in an exotic villa in the South of France, she uncovers more and more parallels between the past and the present, the fictional and the real world – until suddenly she finds that she has become a target herself.
It seems that someone in Eliot’s family doesn’t want the book to be written. And they will do anything to prevent it.