The ISBN, or International Standard Book Number, is a 13-digit (formerly 10-digit) number that is unique to every book published in the world. The breakdown for said number is as follows:
• The first three digits denote the type of publishing. (They're either "978" or "979"; "978" is book publishing.)
• Here it gets tricky, but for American books (and UK, too, I believe), the next digit is a group identifier and indicates in which language the book is published. (The group identifier for English is either "0" or "1.")
• The next three or four digits are assigned by the national ISBN agency (in the United States, that's R.R. Bowker) and are specific to the publisher. Many large publishers have several sets of publisher codes for their various divisions and imprints.
• The next four or five digits are the item number for the individual book and are selected by the publisher.
• The final digit is a check digit, which is the result of a mathematical formula being applied to the previous twelve digits. This is an error detection and prevention measure employed to prevent the ISBN data from being corrupted, mis-copied, &c.
The big, fancy book of publisher codes will set you back nearly $1,000, and there's really no free way of looking up publisher codes on the Internet, but a partial catalog of the group-0 and group-1 publisher codes (read: English language books) can be found here and here, respectively. This is only for those of you with way too much time on your hands.
Questions? Comments? Invective? To the comments with you!