To what degree does library book lending complement book sales, and to what degree does library lending substitute for book sales? I don't think anyone knows for sure. (Well maybe Amazon, but they're not telling.)
With over 40 billion dollars per year of sales at stake, you would think that the US book publishing industry would want to know as much as possible about how those sales are generated. Since US public libraries circulate more items than US bookstores sell, the industry needs to understand the role of libraries in getting people to read and purchase books. Is it small or big? Does the existence of libraries promote sales or hurt sales? How do the equations change when books become digital?
Publishers do a pretty good job of compiling sales data, and they spend a lot of money to figure out what books are selling and who's buying them. According to BookStats, a cooperative study by the AAP and BISG, Americans bought an average of 7.32 books in 2010.
On the library side, there's a bunch of interesting data. IMLS has been compiling a wealth of data about the footprint of public libraries, which is why I can tell you that the average American borrowed 8.1 items from public libraries in 2009. Library Journal has recently published the first installment of results from a fascinating survey of library patrons. (Aside: this study should be made available in every library!) They find that 46% of respondents use the public library less than 2 times per year.
LJ Patron Profiles survey shows a strong relationship between library use and book purchasing. For example, over half of survey respondents report buying a book by an author whose works they'd previously borrowed from the library. That's a huge number, considering that 20% of respondent never go to the library, period. At the same time the survey indicates a competition between reading and borrowing. Respondents who report that they've decreased their use of libraries buy 12.18 books per year, while those who've increased their library usage buy only 10.9 books per year. What we can't tell from the data is cause and effect. With the recession having a wide impact, who's to know whether the folks showing up more at libraries might buy even fewer books if the libraries weren't around!
It costs about 11 billion dollars a year to run public libraries in the US, and libraries work hard to demonstrate their value to the communities that support them. They compile data to measure their activity and the community's return on their investment in libraries. These studies assign much of the benefit of library spending to substitutional activity. For example, a survey by Denver Public Library determined in 2009 that it saved its community $105 million based on the cost to use alternative sources of information, and delivered an additional $5 million by avoiding "lost use", activity that wouldn't have occurred if the library did not exist. (See Public Libraries- A Wise Investment (PDF, 1.4 MB) from Library Research Service)
Do libraries really believe that 91% of their circulations would have resulted in purchases if they didn't exist? There's no hard evidence anywhere that that's true. Every librarian can tell you about patrons who loved a book so much they went and bought the whole series, but there are also users who never buy a book they can get in the library. And what about those readers who never go to the library? Surveys are a cheap way to collect data, but they often don't reflect the real behavior of the people surveyed.
my next post, I'll describe how a cross-industry cooperative approach to book data collection and analysis might provide some light amid the gloom of the reading industry's winter solstice of understanding.