Library book circulation study

Library book circulation per user has no strong, long-run trend.
From 1856 to 1978, library users borrowed from U.S. public libraries about 15 books per user per year. From 1978 to 2004, book circulation per user declined approximately 50%. The growth of audiovisuals circulation, estimated at 25% of total circulation in 2004, accounts for about half of this decline. These figures depend on estimates and disparate samples of libraries with varying circulation and user accounting methods. Nonetheless, these figures are of sufficient quality to suggest that historically established institutions significantly stabilize borrowing behavior.
http://www.galbithink.org/libraries/circulation.htm