The Seattle Public Library (SPL) publishes anonymized, open-access checkout data for every item in its collection, dating from 2005 to the present. To our knowledge, it is the only U.S. library to release checkout data by title with this level of temporal detail: one dataset records exact timestamps for print book checkouts, while another provides monthly aggregates across all formats (e.g., ebooks, audiobooks, print books). Because U.S. book sales data is largely inaccessible outside the publishing industry, SPL’s open checkout data offers a rare and valuable alternative. But how well does it generalize beyond Seattle? Does it reflect book sales? And what can it tell us about readers more broadly?