The LASER Foundation commissioned a report [pdf]from PriceWaterhouseCoopers looking at how one might measure public library impact. The report argues that public libraries have demonstrable impact and proposes "practical and consistent approaches to measuring impact".
I liked this quote from a member of the public:
(My mother) finds the library a lifesaver as (she) meets friends here as well as borrowing books. Also taught mother how to use the Internet to look up information that she may need - really enjoyed it.Not either or but and.