Another source you should consider is Rough Notes. The magazine has been around for a long time and their past articles could be a good source of information for how agencies operated in the past. I don't know how easy it would be to access their archives but would be worth a try. Let me know if you need a contact at the company. They are based in Indianapolis.
I agree with Steve. Any study of the American Agency System should begin (and probably end) with Rough Notes. Within those publications, you will find over 100 years of agency marketing trends, agency management trends, coverage and claim trends, carrier relationship trends and much more. You will also realize that while we are a much more efficient industry today, not all that much has changed. For those of us that have been in this industry for 40+, the Rough Notes library is as facinating as the Smithsonian Institute.
The other source mentioned is the IIABA Future One - Agency Universe Study performed every two years. This is an excellent survey, diving into the numbers and trends surrounding our industry. I am not certain how far back it goes, but it does show trends.