My understanding is that neither a canonical link nor an alternate link would be appropriate to refer back to a source book. If Book A is cloned as Book B and then modified or expanded, the appropriate
canonical link for Book B would be to Book B, not Book A; as for
alternate, it’s a way of signifying a different format (e.g. an RSS feed) rather than a different instance or variant of content. We intend to foreground explicit “version tracking” of cloned books in the new webbook theme, so that the Schema.org isBasedOn property is highlighted both on the cover page in a human-readable format and in the metadata of cloned web books (including tracking a clone of a clone back to the original source). However, this doesn’t appear to solve the issue.
Furthermore, my concern about linking two books explicitly would be that annotations on one book and annotations on the cloned/modified book would get mixed together. The correct approach would be to somehow bring annotations along to the cloned book, but I haven’t the first idea how to do this as they aren’t stored in Pressbooks.