Excerpted briefly here, with much more at the links:
"There is a fear that the more libraries expand their sights to new media (e-books, e-journals, online databases and audiovisual material), printed books will grow obsolete. A 2007 study published in Public Library Quarterly titled "Book Circulation Per U.S. Public Library User Since 1856" by FCC Economist Douglas A. Galbi showed that book circulation per user (the average number of books borrowed by library card-holders) has gone down 50 percent since the late 1970s. What the study doesn't immediately show—but later details—is that the loss is accounted for by unprecedented growth in cardholder numbers coupled with a wider range of items borrowed. Over the past decade, cardholders increasingly use their cards solely to access e-journals, online articles and databases (which don't register as borrowed books) instead of taking out printed material. As Mary Bender of Boston Public Library wrote in an email, "Circulation for downloadable items (audio books, videos, music, and e-books) generally ranks as one of our top five busiest branches in terms of monthly circulation. And the overall circulation for downloadable items is up 32 percent this fiscal year over last."