I’m happy to share how we approach some of these issues for the B.C. Open Textbook Collection. (Much of what we do mirrors what has already been discussed above.) We based our process on practices followed by other open publishing projects and organizations.
We assign both a “Book” ISBN (added to the Print ISBN field in Pressbooks) and “Electronic book” ISBN (added to the Ebook ISBN field in Pressbooks) for all open textbooks and support resources (guides and toolkits) published by BCcampus and placed in the B.C. collection. In addition, we repeat the ISBN information at the end of the Copyright field on the Book Info page for each book so the ISBN info displays in the PDF versions of the books. (Exported PDFs do not pick up the ISBN field information.) The PDFs are used to create print-on-demand versions of these books. We do not add ISBNs for other formats.
New ISBNs are generated when a new edition is released. Each of our publications includes a Versioning History page to track minor revisions between editions. (OpenStax does the same. )
The books in our collection are static, a decision that was made early in the project in response to potential adopting-instructors’ concern that if they assign an open textbook from our collection, they didn’t want it changed during the semester. (The books don’t change except for minor revisions which are noted on the Versioning History page. We are also careful about how we swap out old editions for new. After the new edition is added, we add a note to the older one stating that it will be archived along with the date this will happen. We never remove an old edition during the term in which the newer edition is added so as not to disrupt current use. But rather, select a date at least one month into the next term.)
Adapted versions of the collection’s books, usually produced by our province’s faculty, are not added to the BC collection. Instead, we encourage each instructor’s institution to include these new versions in their catalogue or repository. We do link out to adaptations we’re aware of on that institution’s page in the Post-Secondary Directory.
As for support resources published by BCcampus (directories, guides, toolkits), ISBNs are assigned only to static resources. Ongoing resources, i.e. those that are continuously updated such as the directories (Information Directory, OER by Discipline Directory, Post-Secondary Directory) and the B.C.-specific Pressbooks Guide, are not assigned ISBNs for the reasons cited above.
As for distribution and printing (the reason for ISBNs), we offer print-on-demand services for all our publications.