Two books were found (or "rediscovered") today that have large prominent notices on them that they have been released into the public domain, not under the GFDL. The books have notices on them that "all future contributions to this book must also be released into the public domain". Checking the history, all the edit contributions to these books were from authors who have explicitly stated that all their contributions are released into the public domain, or anonymous authors (who have no legitimate claim to copyright anyway). So what's the verdict? Can a single book on Wikibooks be released into the public domain? Can we tell contributors that they must agree to certain alternate licensing schemes on a per-book basis? And what if a person edits a book or page without seeing that warning first?
This also brings up the topic of cross-licensing. Some books in the past have attempted to cross-license themselves under both GFDL and CC-BY-SA-2.5 (or similar). So long as all text is released under the GFDL, and so long as all authors agree to the cross-licensing scheme, is this acceptable? This is the kind of issue that we have never really come to grips with in the history of the Wikibooks project, and maybe it's something we should be dealing with now.