"Chicago Soul" by Robert Pruter is an excellent starting point. Chapter one is "Chicago at the dawn of the soul era" and it ends with "Chi Sound and the demise of the Chicago Soul Industry". Along the way it takes in chapters on Vee Jay, Chess, Okeh, Constellation, ABC Paramount, Brunswick etc etc. It also looks at smaller entrepeneurs and their labels such as Tarpon, Gerri, Jacklyn and so on.
Highly recommended
ISBN 0-252-06259-0
Godz