S.R. Ranganathan, a university librarian in India, proposed what he called the Five Laws of Library Science in 1931. These laws still hold true, I believe, not just for libraries but also for bookstores and even for private libraries. Here they are:
1. Books are for use.
This seems like an obvious truth, yet for some people books are considered mostly decorative. But even that is a use, I guess. Yet books are intended to be read by somebody at sometime. Most books do spend most of their lives sitting on shelves, but that's OK. Except for the most popular books with long waiting lists, even most library books mostly just sit on shelves.
Unfortunately, out of necessity to create space for new books, public libraries must discard older books, and the criteria they use is mainly their frequency of use. If a book hasn't been checked out for a certain period of time, it may be taken off the shelf unless it is considered a classic and worth keeping for that reason alone.
2. Every reader his or her book.
Some readers may need a long time to find their book, which is why books should be kept on library shelves and bookstore shelves for as long as possible. Somebody will find them eventually.
This law is why bookstores and library must offer a great variety of books, catering to all tastes, all ages all political and social views and, as much as possible, all languages spoken in that particular area. For private libraries, of course, the books usually reflect the owner of the library.
3. Every book its reader.
There are some books that make you wonder: How would anyone be interested in reading this? Yet someone obviously thought the book interesting enough to write, suggesting there must be someone out there who thinks it interesting enough to read. Like the second law, this third law argues for variety.
4. Save the time of the reader.
In other words, organize the library or bookstore in ways that make sense to the browser. Organize nonfiction by subject matter. Organize fiction by author alphabetically. Make signage easy to read and understand. The Dewey Decimal System has long helped public libraries obey this law. Bookstores and owners of private libraries have a more difficult time. A knowledgeable staff is a big help.
5. The library is a growing organism.
This sounds more like a statement of fact or perhaps an ideal than a law to be obeyed. My own private library always seems to grow, unless I make a deliberate effort to weed out books, as I had to do when I sold my house and moved into a condo.
Yet even with limited space, as is true of all libraries and all bookstores (except perhaps for Amazon), the books undergo constant change. New books are published each day, some of which are added to the shelves. Change itself is growth, in a sense, and book collections should never be allowed to become stagnant. So perhaps this is a law, after all.