The number that appears in the barcode on a book is a special type of 13-digit number called an ISBN (International Standard Book Number). An ISBN is assigned to each edition and variation (except reprintings) of a book, so that an e-book, a paperback, and a hardcover edition of the same book each have a different ISBN. As ISBNs were invented before the GS1 barcode system, the GS1 system uses the prefix 978 or 979 to convert them into valid 13-digit barcode numbers.
The allocation of ISBNs is managed in the UK by Nielsens. If you are based outside the UK, go to the ISBN International website, www.isbn-international.org to find your national issuing body. The cost of a single ISBN is £75, with the cost falling per number if you need more of them. The ISBN is used by publishers, booksellers, libraries and retailers worldwide.