The barcode is needed to identify items reliably. In most cases you can find it on the back cover of books. Books can also have an EAN (or ISBN) number. Books that neither have an EAN, IBSN nor a barcode can't be sold.
If you can't find the barcode number on the back of your book, try looking on the first or last page of the book to see if you can find the ISBN instead.
If the barcode is not present on the back of the casing, you may find it within the booklet.
Almost all the time the barcode is to be found on the back of the casing.