Cat. & Reference: ISBNs
History and format
ISBN stands for "International Standard Book Number." It was created in 1966 by a British bookstore, and later expanded to become an official international standard.
ISBN digits are divided into four groups, separated by hyphens. The first group is the country code, which is generally one digit (but which can be up to five digits). 0 and 1 are for English-speaking countries, 2 is for French-speaking countries, etc. This means that the country code generally indicates the language of a book, as well, but not always. For example, my Welsh-language edition of "Harry Potter and the Philosopher's Stone" has an ISBN that begins with a "0," because it was published in an English-speaking country.
The second group is the publisher code, which can be between 2 and 7 digits long. Publishers purchase these codes from the international agencies assigned to distribute ISBNs for their country; they then have the rights to publish all books using that publisher code.
The third group is the title number, which is between 1 and 6 digits long. (The country code + the publisher code + the title code = 9 digits.) The title code consists of numbers assigned to a specific book at the publisher's discretion, usually serially.
The last group is one digit, called a check digit. The check digit is calculated by performing a mathematical operation on the first nine digits, and taking the remainder, which can have a value of between 0 and 10. (If the value of the check digit is 10, that number is written "X," like the Roman numeral, because the ISBN check digit can only be one digit long.)
The purpose of the check digit is to enable computers to "check" that an ISBN has been correctly recorded and transmitted (hence the name). A computer that processes an ISBN can perform the same calculation on the first 9 digits, and check that the last digit is the same. If it is, then the ISBN is probably correct.
ISBNs and "editions"
ISBNs are assigned at the "edition" level of a work, which is basically the level at which multiple copies of the same book look exactly the same, in terms of binding, language, and content. A given book may be reprinted with minor changes (usually errors corrected), but if there are major changes, it's considered a new edition and should have a new ISBN.
If a book is only printed once, there may be only one ISBN which corresponds to it. If a book is reprinted many times by multiple publishers, with multiple editors, and in multiple languages, there may be hundreds of ISBNs which correspond to the same work. (See, for example, the huge block of ISBNs which correspond to editions of Romeo and Juliet on LibraryThing.)
Books that consist of multiple volumes may have one ISBN for the entire series, one ISBN for each volume, or both. In addition, some non-book materials, such as decks of cards or books of stickers, may also carry ISBNs, if they're produced by a book publisher. Audio books and CDs also typically carry ISBNs.
The 13-digit ISBN
In 2007, it was decided that all ISBNs would officially expand from the classic 10-digit format to a 13-digit format by adding the prefix "978." The purpose of this was to make an ISBN also an EAN, or European Article Number, which is the European equivalent of the UPC. In an EAN, the first three digits correspond to a country code. However, ISBNs correspond to books from all over the world (and have their own, internal country codes) so the "978" prefix in a 13-digit ISBN / EAN officially stands for the country "Bookland." (And what a wonderful country that must be.) Adding three digits to the front of the ISBN meant that the mathematical formula which calculated the check digit had to be changed, so the last digit of a 13-digit ISBN is often different from the last digit of a 10-digit ISBN.
10-digit and 13-digit versions of the ISBN are considered equivalent to each other, and both may be found in library records, bookstore databases, and on physical books, themselves. Searching either one should lead you to the same book, although some older systems may have only the 10-digit version recorded. Since the only difference between a 10-digit and a 13-digit ISBN is the prefix and the check digit, it's possible to convert one to the other automatically, and some systems search for both formats, even when only one is entered.
ISBNs and library catalogs
Whether or not an ISBN appears in a library catalog record is at the discretion of the library, itself. ISBNs are typically recorded in MARC records (the complicated data structures on which library catalogs are traditionally built) but they may or may not display on the user side, and users may or may not have an option to search a library catalog by ISBN. In library records, ISBNs are searched and recorded without the dashes that separate the groups, so it's not immediately possible to tell where a publisher block ends and a title block begins, for example. However, ISBNs are still unique, even without the dashes, so eliminateing them doesn't hurt the search function.
Searching multiple ISBNs may lead to the same library record as in the case of a multivolume set where each volume has a separate ISBN, but the librarian has chosen to catalog all of the volumes as one work. In addition, hardback and paperback copies of the same book which are published in the same year will often share the same library record, since they only differ in binding and not in content.
ISBNs and physical books
The most common place for an ISBN to appear is on the back of a book, underneath a barcode. ISBNs may also be found on the inside of a dust jacket or on the inside cover of a paperback novel. They may also appear with other publication information on a page that faces or follows a title page. Some older books have ISBNs, but they aren't printed anywhere on the book, although they may be recorded in library or publisher databases.
ISBNs and LibraryThing
The ISBN is the easiest and most popular way of searching for book information on LibraryThing. If you have a book without an ISBN, it's also possible to search by title and author, although this can lead to way too many results if the book has been published multiple times. In such cases, I usually just go ahead and enter the information manually, because it's too much of a pain to search through all of the editions to try and find the one I want.
As I mentioned before, some multivolume sets can have have a single ISBN for the whole set, as well as ISBNs for individual volumes. Most of these books are quite academic, and unlikely to appear in individual libraries, but some readers may own boxed sets of works such as The Chronicles of Narnia or the Harry Potter books. It's up to the reader if they want to enter these books individually or by the set ISBN. Neither is "wrong," although picking different methods will have somewhat different effects on the set of statistics automatically generated. (If you enter all of the Chronicles of Narnia under one ISBN, then it will count as one only book by C.S. Lewis under your author cloud, for instance.) Personally, if the books in a set are commonly sold separately, I'd probably enter them into LibraryThing separately, as well.
Limitations of the ISBN
ISBNs are very useful for publishers and booksellers, but they can fall flat for readers, because they identify works at a level that's too specific or not nuanced enough. For example, if you and I say we have both read Jane Eyre, but later discover that we read different editions, we are still likely to say we have read the same book. In addition, if we both liked our respective editions of Jane Eyre, and then discover that we both liked different editions of The Eyre Affair, we're likely to say, globally, that people who like Jane Eyre will probably also like The Eyre Affair, not that people who liked the Signet Classics paperback version of Jane Eyre will also like the Viking paperback edition of The Eyre Affair, while people who liked the Random House paperback edition of Jane Eyre will also like the Hodder & Stoughton hardback edition of The Eyre Affair.
There are times when we may care about which particular version of a book someone has read, in terms of reading a book in the original language vs. reading it in translation, reading the abridged or unabridged version, or reading a version with a particular editor and set of notes. However, none of these distinctions corresponds exactly to the publisher's idea of ISBN. Furthermore, there are higher-level relationships between works, such as the relationship between Ten Thousand Acres and King Lear or between Grendel and Beowulf, which can be difficult to express in formal bibliographic terms.
LibraryThing has taken an interesting approach to the first problem, which is to allow its members to use their own judgment in combining different editions of the same book. This also allows the combination of the statistical information related to that book, which provides more accurate information regarding global popularity of individual titles and authors, and the relative popularity of titles with respect to each other.
A last problem is that not every published work carries an ISBN, either because the book was published before ISBNs existed, or because the publisher didn't want to bother to apply for one. I've frequently noticed this problem with a lot of Church publications, including lesson manuals and copies of the Book of Mormon. I can only assume that since these materials are typically distributed through Church-owned media outlets (and often for free) that the powers that be decided not to bother with paying for ISBNs. When I have to enter such materials into LibraryThing and I have good reason to think that the materials are already on the site, I'll often to a title search, first, and then click the "add to your catalog" button instead of bothering to enter the information by hand.