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Short Answer
For print books sold through bookstores or wide distribution, yes. For ebooks, often no. An ISBN identifies one edition of a book in one format. Many ebook stores, including Amazon's Kindle Store, assign their own identifiers, while print distribution to bookstores and libraries requires an ISBN.
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What It Is
What an ISBN is
An International Standard Book Number is a 13-digit identifier for a specific edition and format. Your paperback, hardcover, ebook and audiobook each need their own ISBN if they have one at all. In the US, ISBNs are sold by Bowker, the official agency.
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Costs
What ISBNs cost in the US
Option Price Notes
1 ISBN from Bowker $125 You're listed as publisher
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Isbns From Bowker $295 Best Value For One Book In Several Formats
Free platform ISBN $0 The platform is listed as publisher; usable only on that platform
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