The limitation is regrettable, because ISBNs are just a matter of privilege/means. My book would have one if they were free, but they aren't. So, I went with an ASIN and it would be nice to be able to include my book in my profile. Especially since the vast majority of Medium's content is made at no cost to the platform. Allowing us an easy way to promote our books that are available on known, established retail sites is a bad look, as the kids say.