Open those specific files and try again.
Thanks Barb. that worked...
this software is kind of quirky sometimes, no?
But, unavailable fonts always prohibit a book update. Ideally, you should open that file, and use the Font/Replace Font panesl to find and eliminate them. But before you start tearing your hair out, I'll warn you that it doesn't always work. The message will appear and there's no fixing it, other than redesigning the doc from scratch. So yes, quirky. But opening the troublesome file(s) is a consistent workaround.