The following is a Plus Edition article written by and copyright by Dick Eastman.
“ISBN” stands for “International Standard Book Number.” An ISBN number is an ISO standard and normally is found in all books published in the United States since 1970 and on many books published in other countries as well. Technically, an ISBN number is not a requirement for any book; you may publish books without such a number. However, experience has shown that an ISBN number is required if you want the book to be listed in the many indexing and cataloging systems available. Also, an ISBN number is required for all books that are to be sold by Amazon, Barnes and Noble, and most any other major bookseller. These booksellers use the ISBN numbers to order, inventory, and track books. If your book or ebook includes an ISBN number, it will also be listed in Bowker Books in Print®, which is used by all the major search engines and most bookstores and libraries.
Only the smallest self-published and self-marketed books can survive without ISBN numbers.