Where they come from

One agency per territory holds the register. In the United States that is Bowker; in the United Kingdom, Nielsen; in Canada and several other countries the national library issues them free to residents.

Buying singly is always the worst value. Agencies sell blocks, and you will need more numbers than you think.

How many numbers you actually need

One ISBN per format, not per book. Paperback, hardcover, ebook and audio book are four separate numbers. A second edition with real changes is another.

A film book is a video work, not a print edition, so it does not take an ISBN. It is identified by its own metadata and, in this house, by its entry in the register of provenance.

Free storefront ISBNs — the honest trade

A free ISBN from a retailer costs nothing and works. The trade is that the retailer's imprint appears as the publisher of record, and the number is usually tied to that one storefront.

If you intend to sell through bookshops, libraries or your own imprint name, buy your own. If you are publishing one ebook to one storefront, the free number is not a mistake.

Asked and answered

Do I need an ISBN for an ebook?

Not at Amazon, which issues its own identifier. Most other storefronts and every library system prefer a real ISBN.

Does an audiobook need its own ISBN?

It needs its own identifier, and an ISBN is the cleanest one. It is a separate format, so it cannot share the print number.

Does a film book need an ISBN?

No. It is a video work and carries video metadata instead.