Pay-Off in Blood — Brett Halliday

A desperate doctor hires Mike Shayne for a case that starts with blackmail and ends with murder

Back when Mike Shayne was living in New Orleans, his best friend, Tim Rourke, got shot on Miami Beach. Rourke, star reporter for the Miami News, was working on the story of his career when gunmen pumped him full of lead. It took quick work by the brilliant Dr. Ambrose to save him. So when the good doctor finds himself in mortal danger, Rourke repays the debt the only way he knows how: He calls Mike Shayne.
 
Ambrose's trouble started at the gambling tables. Now, he's in the clutches of a blackmailer who intends to bleed $1,000 from him every month until the poor surgeon drops dead. At stake are Ambrose's practice, his marriage, and his life. In an act of desperation, he asks Shayne to help him make the blackmail payoff, which the detective outright refuses to do. But when the doctor's trouble turns into a murder case, Shayne will find himself unable to turn his back on the man who saved his friend's life.
 
Pay-Off in Blood is the 41st book in the Mike Shayne Mysteries, but you may enjoy reading the series in any order.