Now that we've looked at how back-of-book indexes endeavor to organize and present the information found in a book, we can consider organizing books themselves. It's quite astonishing, how many people go to libraries and bookstores who never seem to stop to think about how books end up on particular shelves in particular areas. There is no magic Book Placement Fairy!
Let's consider the problems we're trying to solve for a moment. A library has a lot of books, on which ordinary inventory-control processes must operate. So librarians as well as patrons must be able to locate the specific book they're after based on information they have about the book, and once they have the book in hand they must be able to reassure themselves they've found the right book.
What information should librarians capture about the book in order to make this possible? What should they put on the book, and where should they put the book, to make it easier? (Before you answer, consider a book with multiple editions, or purchased in multiple copies.)
The next problem we'd like to take a stab at is enabling patrons to discover useful or interesting books based on the books' physical location. This hasn't always been a desideratum: consider books chained to lectrums, closed stacks, and the more recent phenomenon of offsite book storage. Still, just about any library with open stacks wants the physical location of a book to be a Hansel-and-Gretel breadcrumb trail, leading readers almost invisibly to related materials.
So, just to throw one oft-mentioned possibility out right away, organizing books by cover color is probably not the way to go here… It's also worth mentioning that physicality sometimes interrupts the perfect vision of library classification: "oversize" storage is necessary for books that don't fit on the regular shelves alongside what would otherwise be related materials.
We do have one important constraint to consider: a book is a physical item that can only be shelved in one place. (Multiple copies of a book are just about always shelved together.)
What librarians do to identify books and put related books near each other is called "classification," and as I hope you've guessed by now, it usually involves determining what the book is "about" and what other books are "about" the same or similar things. The phenomenon of bringing together related information packages is called "collocation" in librarian-speak, and is an important principle underlying classification.
(There are exceptions to "aboutness" as the underlying criterion for classification. For example, many public libraries shelve fiction by genre and author rather than "aboutness," and there are longstanding arguments about how best to shelve biographies and memoirs.)
Classification is not an exact science; for one thing, it tends to be contextual. The same book may be in two very different places in different libraries, depending on the contours of each library's collection and the predilections of its patron base. Still, librarianship has developed several classification schemes to assist with this problem… and I'll be discussing some of them in my next post on the subject.
In the meantime… go to your local library and scrutinize the shelves for a bit.