The Quick Red Fox by John D MacDonald

Travis McGee is asked by a movie actress to find out who is trying to blackmail her, a second time, over pictures at a party she does not want the public to see.  He agrees and finds himself ‘saddled’ with the actress’ personal secretary.  They start tracking down the other people who were at the same party to find out which were behind the photo-taking.  They go from Miami to a rehab and decide that person is harmless to the situation.  Then they go to New York to check on another red herring.  Then they go to the scene of the crime in California and discover why it was so easy for the photographer to take the pictures from about 100 yards away.  Then they go to Oakland and track down the photographer, and the daughter who helped him.  Only the photographer has been murdered and his assistant has fled town.  They track another member of the party to a ski lodge and discover he is just a braggart but did not know about the photos.  They find another member of the party in Las Vegas, recovering from multiple bad relationships.  Then they hear that she has been killed.  They track down the last member of the party with his wife in Phoenix.  During a horse ride, he is thrown from his horse and dragged.  Only his head has been bashed-in just like the other deaths.  Turns out the man’s wife did all the murders to try to cover up her quest for the man’s riches.