W.E. Johns 
The Unknown Quantity [EPUB ebook] 

This excellent, fast moving and exciting adventure begins with Nigel Deane, known as “Gunga”, summoning his brother Guy, to discuss the death of their older brother Peter. Pete was a journalist and Gunga is convinced he has been murdered because of a letter that Pete sent to Gunga the night of his death. Pete was investigating a mysterious gang, who identify each other by a 1904 Victorian Penny and whose members are only known by numbers. Pete was killed in a car crash at a place called Blandford. Gunga and Guy resolve to kill everyone involved in Pete’s death and bring this criminal organisation down, come what may…