ISBN stands for International Standard Book Number. An ISBN is a unique 13-digit number (previously a 10-digit number) used as the basis of inventory systems by bookstores and book wholesalers.
The ISBN system is used internationally. Every country has one agency authorized to sell ISBNs to publishers in that country. The agency might be an office within the national library, or a company in the local publishing industry. The U.S. agency responsible for ISBNs is R.R. Bowker Company, which also publishes the database Books in Print.
If you do not plan to sell your book through bookstores or book wholesalers, then most likely you do not need an ISBN. If you do want to sell through bookstores or book wholesalers, you will need an ISBN.
Even a small independent bookstore in your neighborhood is unlikely to carry your book if it has to be handled outside their normal inventory system.
However, you do not need an ISBN to sell your book through Amazon, through a club website, at your speaking engagements, and so on.
As mentioned above, publishers are expected to assign separate ISBNs to different editions of a book. However, the same ISBN is used for different printings of a book. What's the difference?
Suppose you start off having 200 copies of your book printed. You sell them all, and decide to have another 200 printed. The second batch has identical content to the first. Do not assign a separate ISBN to the second batch, because the content and format are the same. The second batch of books is not a second edition, it’s merely a second printing.
Suppose you write a book on the current political situation in Country X. After a few years, you have new material to incorporate. You rewrite your book, adding the new material here and there, or you add a new chapter at the end. This is a second edition: the content is clearly different from the first edition. You should assign a separate ISBN and add “second edition” or “revised edition” to the end of the title.
What if your revisions are minor? If you’re simply correcting a few typos that got through your proofreading process the first time, don’t call it a second edition. A new edition of a book means that the content has been revised or expanded in some way.
ISBNs can be purchased singly or in blocks. The current price structure (September 2014) is: $125 for 1 ISBN; $275 for 10 ISBNs; and $575 for 100 ISBNs.
Obviously, it is more cost-effective to buy a batch of ISBNs rather than one at a time. And, even if you are self-publishing only a single book and you never plan to write another, you may need more than one ISBN.
This is because ISBNs are meant to identify a unique combination of content and format. Publishers are expected to assign separate ISBNs to:
Think of this from the point of view of a book-store handling a special order for a customer. If you used the same ISBN for a standard edition and a large-print edition, it would be hard for a bookstore to be sure they’ll be getting the right item for a customer who needs the large print.
ISBNs are not meant to be transferred from the purchaser to another customer. There are ISBN re-sellers on the Internet, other than the Bowker company, which purchase large blocks of ISBNs and sell them individually at a lower price than it would cost to get them from Bowker. These re-sellers are violating the terms of sale they agreed to when they purchased the ISBNs. The MSU Libraries will not print books with ISBNs obtained in this way.
Buy ISBNs at the R.R. Bowker Company website.