The Sinister Scourge was the eleventh adventure of the mysterious government operative and master of disguise ("The Man Of A Thousand Faces") known only as "Secret Agent X." The plot concerns a gang, led by the evil mob kingpin Karloff (named after the movie villain, I suppose) that's seeking to flood the nation with narcotic drugs. It was originally published in the January 1935 issue of the Secret Agent X pulp magazine and was written by Paul Chadwick under the "house" pseudonym of Brant House.
This 1966 reprint by Corinth Publications (known primarily as a porn publisher) was the seventh and last volume in their series of paperback books reprinting Secret Agent X pulp novels. The book is listed on the front cover as "Secret Agent X Book No. 07" even though the story was actually the eleventh in order as published in the original pulps.
From the teaser page:
"FLAMING PHOSPHOROUS ••• frustrated and imperiled the supersleuth, Secret Agent X, wherever he turned in his search for the dire criminal mastermind whose gigantic narcotic ring threatened to enslave the nation. All X's genius at disguise and subterfuge seemed impotent against the baffling plot of ••• THE SINISTER SCOURGE."
From the back cover:
"A Flood of Narcotics was sweeping the Nation ... and it was free! - the only price, hopeless addiction. Into this bewildering maze of crime seemingly without profit, Agent X plunged headlong, seeking the sinister motive behind the deluge of dope. His life in continuous peril, he threaded his wily way toward the monstrous, mysterious kingpin whose plot threatened to cripple the entire nation!"