An ISBN is an International Standard Book Number. Every book sold anywhere in the world should have a unique ISBN, meaning that you can find any book in the world simply with that one number, which never duplicates. ISBNs are assigned by R.R. Bowker in the United States and by a variety of agencies worldwide. Publishers apply to these agencies to get a range of numbers with a unique prefix which varies in length. Because of mergers and acquisitions, some publishers have several, or even dozens of prefixes.
Publishers choose what numbers to assign to which books following their prefixes.
ISBN have no relationship with subject matter, nor do the dashes mean anything. Every ISBN should be unique. Occasionally, ISBNs are reused for books that have long been out of print, but this practice is generally discouraged. ISBNs are 10 characters long. A checksum is a digit that is the product of a series of mathematical operations on the first digits that help doublecheck that the other digits are correctly entered into a system when typed manually or barcode scanned.