ISBN

The International Standard Book Number (ISBN) is "a numeric commercial book identifier that is intended to be unique." What it identifies is narrower than writers often assume: a different ISBN is assigned to each separate edition and variation of a publication, but not to a simple reprinting, so the ebook, paperback and hardcover of one book each carry their own number while an unchanged reprint keeps the old one. It is an identifier of editions, not of works.

A 13-digit ISBN has five parts: a GS1 prefix (978 or 979 so far), a registration group, a registrant, a publication element and a check digit. The 13-digit form applies to numbers assigned on or after 1 January 2007, and a 10-digit number from before converts by prefixing 978 and recalculating the check digit. Numbers come from national registration agencies, and publishers buy or receive them in blocks. Most countries do not legally require an ISBN, but distribution effectively does: major booksellers handle books that carry one, and Amazon requires an ISBN, EAN or JAN to list a book, which is why a self-published or serial-collected edition normally ends up with one. Privately published books sometimes appear without.