Here's a link to an interesting article about ebooks and ISBNs:
http://ebooks.epicauthors.com/?tag=isbn
And also a link to a Mobileread thread on the same topic:
https://www.mobileread.com/forums/showthread.php?t=63578
Nothing to do with Calibre, but just some background on the ebooks/ISBN issue to explain my Calibre question.