Carroll John Daly 
The Hidden Hand [EPUB ebook] 
Race Williams #19 (Black Mask)

Race Williams had run across criminals before, and a few shots to the head always took care of such threats. But how can Race deal with four separate rogues at once? And what of their ultimate leader, The Hidden Hand? Story #19 in the Race Williams series.
Carroll John Daly (1889–1958) was the creator of the first hard-boiled private eye story, predating Dashiell Hammett’s first Continental Op story by several months. Daly’s classic character, Race Williams, was one of the most popular fiction characters of the pulps, and the direct inspiration for Mickey Spillane’s Mike Hammer.