Updating DITABook chapters from xml

Using DITABook looks like the best option right now for moving our doc sources to xml, but still producing the high-quality PDF documents we're used to. But, I'm a bit puzzled, and haven't found any discussion of updating the chapters in a DITABook from the XML sources.

The chapters are saved with a .xml.fm extension, which suggests that at least the identiy of the xml origins are preserved, and so could be used for updating the chapters should the underlying sources be changed. But, I'm finding no way to do this except to either recreate the book (which isn't too bad, but does entail rework) or at least delete and re-add the chapter that has changed.

Am I overlooking something, or is that really the idea, that books are created once at the end of a production cycle?

Thanks in advance,
Bruce