Murder by the Pack by Carl G. Hodges (Original Novels Foundation, 1960?)

Found in a dusty corner of the Green Shed:

They got to Cochran just outside the safety of his home.  And when Bob Ruff got there his pal was sprawled out on the sidewalk—dead with nine bullets in his body. 

The tough shamus owed his life to the murdered man and he vowed this was one gang-killing that was going to be avenged. 

Ruff figured he was going up against vicious opposition, but he didn't know just how vicious until he met a girl on the run. 

Cherry Morgan knew more about the rackets than was good for her health.  And when Ruff learned the score he knew that every moment they both lived from then on was on borrowed time!

Though a lot of these digest sized novellas were published in Australia in the 1950s and 1960s, not many of them have survived.  They appear to have been throwaway material—quick and easy reads for commuters on their journeys to and from work.  I read this on the bus this morning and it was just the thing for the journey: an absorbing page turner that was small enough to stuff into a bag or a pocket when I reached my stop!