Hit Them Where it Hurts — James Hadley Chase

Dirk Wallace is handed a simple assignment - find out who's blackmailing Mrs Thorsen's daughter. Before long, Wallace realises that he's up against some very organised crime. And when things turn nasty, there's only one thing to do. Wallace and his assistant quit the Acme Detective Agency and go it alone. There's a score to be settled. And Dirk Wallace doesn't want to have to play by the rules.