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Tim Coates, of Bilbary, made the statement that "twice as many books are borrowed in the US as are sold by booksellers". This factoid made it into at least one report of the session, on the role of libraries moving forward. I recently worked on compiling some data about this, and while the effect of the statement is still valid, it's not accurate. And for all I know, Coates may have misinterpreted something I wrote, so I feel that I should participate in fixing this data bug.
In 2009, the most recent year for which real data is available, US public libraries circulated 2.44 billion items, according to data from IMLS. That's a lot. But not all of them were books. In Ohio, 40% of the public library circulation that year was non-book material, or in other words, audio and video. If we assume the same fraction for the entire US, we're left with a book circulation of about 1.45 billion. US public libraries spent $889 million on print materials in 2009, or 61 cents per book circulation.
The BookStats 2010 survey reported publisher net unit sales of 2.57 billion. Trade books comprise about half the dollar volume, $13.94 billion. So if we compare public library circulation with trade book unit sales, they're roughly equal in units. If we compare the public library spend with the trade book total revenue (offset by one year), we see that public libraries make up about 6.4% of the market. If you add the K-12 education sector to the total (34% of library circulation is "juvenile" literature), the library share drops to 4.6%
These data-based numbers are worth keeping in mind when we think about the economic impact of library lending on book sales. We shouldn't single out Coates; I've heard un-sourced numbers quoted that are all over the place. There's so little hard data available that these real numbers are all that more precious.