An ISBN is a special number given to a book when it is published, so shops, libraries, and computers can tell that exact book apart from every other. The letters stand for International Standard Book Number. You usually find it on the back cover, near the barcode. A hard cover and a paper cover of the same story can have different ISBNs, because they are sold as different products. A new edition can get a new number too. Magazines and journals use a sister number, an ISSN, not an ISBN. Though actually, not every printed thing has one — a school worksheet, a family photo book, or a tiny pamphlet may never get a number. And the barcode you scan is not itself the ISBN; it is a machine picture of that number (with a little extra). In speech people just say the four letters. It is a shop-and-library word, not a poetic one.