Chicago Serenade

In this 39th Cyrus Skeen novel, set in April 1930. Skeen flies to Chicago to help a former Yale classmate, who calls him at the office about his problem. After listening to his friend's problem, Skeen decides to fly to Chicago to look into the conflict. He arrives by plane in Chicago a day later from Mills Field in San Francisco and goes directly to the Board of Trade Building where his friend has the main office of a radio station he owned, and over which several gangs were fighting for control of - for whom?