Squealer [aka The Squeaker], The by Edgar Wallace (1875 - 1932)

In 1920s London, a mysterious informant - known only as the Squealer - is systematically betraying London's criminal class to the authorities, leaving a trail of murder and mayhem in his wake. Then an attempt is made on the life of Chief Inspector Barrabal of Scotland Yard, as the unexpected twists and turns hurtle towards a startling conclusion.

From the golden age of detective fiction, Edgar Wallace was a prolific author with over 170 novels to his credit, including King Kong. More than 160 films have been made of his works, and a publisher once claimed that a quarter of all books read in England at the time were written by him. Summary by ASharma