This applies to writers who will be publishing direct, not ones going through the traditional method of licensing their work under contract to a publisher. In the latter case, the publisher handles the ISBN.
Laura J. Dawson, who is the expert on book identification matters — which includes the ISBN — holds a one-hour chat on Twitter just about every Friday at noon Eastern Time. Pop the search term hashtag #ISBNHour into a search engine or Twitter search to see previous discussions.
Today I asked her why a writer should pay for their own ISBN (which is what is required, for example, to use BiblioCore as a distributor to the iBookstore). She replied:
And then someone else chimed in with an angle I hadn’t figured:
And she agreed:
Then I asked if an eBookstore couldn’t put their own ISBN over someone’s book. She replied:
But I was being a PITA with another side-agenda there and asked her again why one eBook wound up with two different ISBNs. She replied:
And there was this fitting end:
Let me also explain that originally I did not see any need for a writer to have an ISBN if he was selling his own work. Laura J. Dawson was the one to show me otherwise. So, this really matters to every direct-publishing writer.
Update: DOH! It’s Laura, not Linda. I’ve fixed all that. And now I give up. From now on, I’ll just think of her as “LJ.” She’ll really hate me now.