[Click on the image to expand.]
The above is from a series of surveys, conducted by JVA Consulting, LLC, at the end of 2011, and during the first part of 2012. JVA questioned more than 3959 community and staff members (the results are still being tallied) about a host of library issues. The key takeaway from the above slide was this: "Borrowing patterns vary greatly by subgroup. However, purchasing and obtaining books elsewhere remains equal among groups."
Note: in this case, "non-user" means "uses once a month or less." "Non-supporter" means "would be unlikely to vote for a library funding increase."
Does using the library mean that you buy more books? In Douglas County, apparently not. Regular readers are no more likely to buy books than non-library users. On the other hand, they are no less likely. Using the library doesn't steal anybody's sales. The library probably does have an effect on which books they buy, though.
And note a third point: while we didn't stop anybody from buying books, the library bought a heck of a lot of them on top of those sales. P.S. Now that we have added a "click to buy" button in our catalog, as well as a recommendation engine for new titles, I would anticipate that library users would buy MORE than others. But we'll see.