ISBN or International Standard Book Number is a unique numeric commercial book identifier that began use in 1970.
Numbers assigned after 2007-01-01 are 13 digits long and known as ISBN-13 while those prior to that date are 10 digits long and known as ISBN-10. Different ISBNs are assigned to each edition and substantial variation of printed books, e-books, and even audio books (except for reprintings when there are no substantive changes) thus a hardback, paperback, .pdf, .mobi (e-book), and .epub (e-book) of the same book should all have different ISBNs.
ISBN-13 numbers are comprised of 5 groups of digits each typically separated by a dash. They indicate, respectively from left to right: Bookland Country Code, Group, Publisher, the specific title, and a check digit.
ISBN barcode generator
gRegor Morrill marks up books he wants to read with an ISBN URL that redirects to Amazon.com using https://github.com/mwhite/php-isbn/blob/master/Isbn.php