ISBN

The ISBN (International Standard Book Number) is a 13-digit identifier assigned by a national agency (in France, Afnil). Each distinct edition of the same book, whether paperback, pocket or ebook, in principle has its own ISBN.

For a self-published author, an ISBN is only mandatory in certain cases: sales in physical bookstores, multi-platform distribution outside Amazon, or legal deposit as a publisher. Many ebooks sold only on Amazon KDP or by direct sale (Gumroad, Payhip) don't need one.

In France, the ISBN is free for authors who self-publish (apply through Afnil), provided they declare themselves as the publisher of their own work.