The Monster of Florence by Douglas Preston and Mario Spezi.
It might be difficult to believe, but this is not a work of fiction. The story centres on Douglas Preston, an American writer who moves to Italy to work on his latest novel - but by sheer coincidence, the villa he rents is adjacent to one of Italy's most infamous crimes: one of several brutal serial murders attributed to "The Monster of Florence", Italy's answer to Jack the Ripper.
The book is divided into two halves: the first recounts the original string of murders from the point of view of Mario Spezi, a journalist working on the case. The second half follows Preston's obsession with the case and his work with Spezi to try and unmask the killer twenty years after the final crime. Along the way, they encounter an ambitious prosecutor who believes he has uncovered a vast conspiracy of murders and satanic rituals that threatens to destabilise the entire fabric of Italian society, and will stop at nothing to prevent Preston and Spezi from embarrassing him.
For our American viewers, he's the same prosecutor who attempted to charge Amanda Knox with the murder of Meredith Kirchner.