The Crimson Circle by Edgar Wallace.
First published 1922; cover shown is the 1955 Hodder & Stoughton Yellow Jacket paperback edition. Cover artist unknown. 190p.
"If it had not been for the accident which saved a criminal from the guillotine, there would have been no Crimson Circle mystery. Instead, some years after this lamentable miscarriage of justice, there appeared in England a ruthless and mysterious blackmailer, who, by careful organisation and a display of remarkable patience and energy, gathered around him a large number of assistants, all of whom were unknown to one another; an enigmatic someone who signed himself the Crimson Circle."