If I handed you a book and asked whether it was in copyright or in the public domain, you'd probably turn to the copyright page first. Unfortunately, a copyright page can't answer that question definitively -- at best, it could tell you when the book in your hands was published, and who owned the rights to it at that time. Ownership can change, though: rights revert back to authors, and after enough time has passed, the book enters into the public domain, letting people copy and adapt it as they wish.So how much time is "enough"? It varies , often depending on the country, on when the book was published, and whether the author is living. For U.S. books published between 1923 and 1963, the rights holder needed to submit a form to the U.S. Copyright Office renewing the copyright 28 years after publication. In most cases , books that were never renewed are now in the public domain. Estimates of how many books were renewed vary, but everyone agrees that most books weren't renewed. If true, that means that the majority of U.S. books published between 1923 and 1963 are freely usable.How do you find out whether a book was renewed? You have to check the U.S. Copyright Office records. Records from 1978 onward are online (see http://www.copyright.gov/records ) but not downloadable in bulk. The Copyright Office hasn't digitized their earlier records, but Carnegie Mellon scanned them as part of their Universal Library Project, and the tireless folks at Project Gutenberg and the Distributed Proofreaders painstakingly corrected the OCR.Thanks to the efforts of Google software engineer Jarkko Hietaniemi, we've gathered the records from both sources, massaged them a bit for easier parsing, and combined them into a single XML file available for download here There are undoubtedly errors in these records, but we believe this is the best and most comprehensive set of renewal records available today. These records are free and in the public domain, and we hope you're able to use them to determine the copyright status of books that interest you.At Google, we're committed to making as many books available online to users as possible while respecting copyright, and this is one example of that commitment. Watch this space for more to come.