Frequently, historians want access to circulation data. This is not easy to get but contemporary circulation data for many papers can be found at the Audit Bureau of Circulation's website. The ABC's historical data reaches back as far as 1914 but is only available to Audit Bureau members.
For more information, see this helpful guide from the University of Minnesota:
If you are researching the history of a particular book or circulating publication like a journal or newspaper, here are some resources to get you started.