Direct answer: An ISBN (International Standard Book Number) is a unique 13-digit code assigned to a specific edition and format of a book — it identifies the exact publisher, edition, and binding, which makes it more precise for searching than a title alone.
Reading the number itself
A modern ISBN starts with a 978 or 979 prefix (identifying it as a book), followed by a registration group (often a country or language area), a publisher code, a title code, and a final check digit used to catch transcription errors.
Why ISBNs matter for searching
Two paperback and hardcover editions of the same novel have different ISBNs. If you're trying to find a specific edition — say, the one with a particular cover or translator — searching by ISBN instead of title avoids ending up with the wrong version.
Where to look one up
The ISBN is printed on the copyright page and usually on the back cover barcode. Open Library, Google Books, and most library catalogs let you search directly by ISBN and instantly pull the exact edition's metadata, including publisher, page count, and cover image.
Older books without an ISBN
ISBNs weren't standardized until 1970, so books published earlier often have no ISBN at all. For those, search by title, author, and publication year instead, or use the older 10-digit Library of Congress Control Number if available.
Is an ISBN the same as a barcode?
The barcode encodes the ISBN visually for scanning, but the ISBN itself is the underlying number, which also appears in text on the copyright page.
Do ebooks have ISBNs too?
Most do, and it's typically a different ISBN than the print edition of the same book, since ISBNs are assigned per format.
Can two different books share an ISBN?
No, ISBNs are designed to be globally unique to one specific edition of one specific book.