The Monster of Florence, By Douglas Preston with Mario Spezi

After moving to Florence to write a novel, the American crime writer Douglas Preston made contact with Mario Spezi, a celebrated local crime reporter, to help him understand how Italian policemen tick. The novel went nowhere, but thanks to his friendship with Spezi he found himself with an inside track on the investigation into the horrific "Monster of Florence" murders.

Between 1968 and 1985 eight couples were murdered in the countryside near Florence after making love in their cars. By the time Preston arrived in 2000, Spezi was the world authority on the crimes. He was first on the scene of one murder, and on first-name terms with everyone involved. Every time police got a likely monster behind bars, another couple would be butchered.

Spezi developed his own ideas. The key was the murder weapon: the same pistol each time. An anonymous tip, and a lucky mistake in the forensic department, enabled the police to discover that the same weapon had been used to kill a married woman and her lover in their car in 1968. That had seemed an open-and-shut case of marital jealousy, for which the cuckolded husband was serving life. But it was more complicated. Police began to believe that the monster was somehow linked to the Sardinian immigrant who had been jailed for the crime.

Spezi thought the same, but felt the police had failed to pursue that line with sufficient rigour. The man he believes, for persuasive reasons, to be the monster - named, pictured and interviewed in this book - only came briefly within the frame. The last murder was committed in 1985 but, lacking a culprit, the investigation continued to steam ahead under different chiefs, becoming increasingly baroque and bizarre.

A seedy farmer called Pietro Pacciani who had murdered his wife's lover and been jailed for raping his daughters was convicted of the murders, but later acquitted for lack of evidence. A new investigator claimed that a satanic conspiracy was behind the crimes, the "evidence" produced by a psychic. The case fell victim to Italy's most popular pastime, "dietrologia", or conspiracy theory. Prosecutors fed fantastic theories to grateful journalists; the banal truth was the last thing on anyone's mind.

Spezi and Preston took a stand against this nonsense, provoking the ire of prosecutor Giuliano Mignini (also the man prosecuting Amanda Knox for the murder of Meredith Kercher). Mignini put Preston under investigation and Spezi in jail, claiming insanely that Spezi himself was the monster and confining him to isolation for five days. Their gripping story gives a deeper insight into the murky workings of Italian justice than any book in English for some time.

Peter Popham is Rome correspondent of 'The Independent'

Click here to purchase this book